[Nexthop] Add CMake test infrastructure for FBOSS Image Builder

[raghav-nexthop]

Pre-submission checklist

• [X ] I've ran the linters locally and fixed lint errors related to the files I modified in this PR. You can install the linters by running pip install -r requirements-dev.txt && pre-commit install
• [ X] pre-commit run

Summary

Integrate distro_cli Python unit tests into the FBOSS CMake build system to enable automated testing in CI/CD pipelines.

• Add FbossImageDistroCliTests.cmake to discover and register Python unit tests
• Configure test discovery for distro_cli modules (builder, cmds, lib, tools)
• Update root CMakeLists.txt to include distro_cli test suite
• Enable distro_cli tests in GitHub Actions workflow
• Update distro_cli README with build and test instructions

The CMake configuration automatically discovers all *_test.py files and registers them as CTest targets, allowing tests to run via 'ctest' or 'make test'.

Test Plan

git checkout raghav.distro-cli-cmake-part1 && cd fboss-image/distro_cli && python3 -m pytest tests/ -v
Switched to branch 'raghav.distro-cli-cmake-part1'
=================================================================== test session starts ====================================================================
platform linux -- Python 3.10.12, pytest-8.4.1, pluggy-1.6.0 -- /usr/bin/python3
cachedir: .pytest_cache
rootdir: /work/raghav/private-fboss/fboss-image/distro_cli
collected 32 items                                                                                                                                         

tests/build_test.py::TestBuildCommand::test_build_all_stub PASSED                                                                                    [  3%]
tests/build_test.py::TestBuildCommand::test_build_command_exists PASSED                                                                              [  6%]
tests/build_test.py::TestBuildCommand::test_build_specific_components_stub PASSED                                                                    [  9%]
tests/cli_test.py::ValidatePathTest::test_validate_path_converts_to_path PASSED                                                                      [ 12%]
tests/cli_test.py::ValidatePathTest::test_validate_path_existing_file PASSED                                                                         [ 15%]
tests/cli_test.py::ValidatePathTest::test_validate_path_nonexistent_file_raises PASSED                                                               [ 18%]
tests/cli_test.py::ValidatePathTest::test_validate_path_nonexistent_file_without_check PASSED                                                        [ 21%]
tests/cli_test.py::CommandGroupTest::test_add_command_to_group PASSED                                                                                [ 25%]
tests/cli_test.py::CommandGroupTest::test_add_command_with_arguments PASSED                                                                          [ 28%]
tests/cli_test.py::CommandGroupTest::test_command_group_creation PASSED                                                                              [ 31%]
tests/cli_test.py::CommandGroupTest::test_command_group_with_arguments PASSED                                                                        [ 34%]
tests/cli_test.py::CLITest::test_add_command PASSED                                                                                                  [ 37%]
tests/cli_test.py::CLITest::test_add_command_group PASSED                                                                                            [ 40%]
tests/cli_test.py::CLITest::test_add_command_with_arguments PASSED                                                                                   [ 43%]
tests/cli_test.py::CLITest::test_cli_creation PASSED                                                                                                 [ 46%]
tests/cli_test.py::CLITest::test_cli_with_verbose_flag PASSED                                                                                        [ 50%]
tests/cli_test.py::CLITest::test_cli_without_verbose_flag PASSED                                                                                     [ 53%]
tests/device_test.py::TestDeviceCommands::test_device_commands_exist PASSED                                                                          [ 56%]
tests/device_test.py::TestDeviceCommands::test_getip_stub PASSED                                                                                     [ 59%]
tests/device_test.py::TestDeviceCommands::test_image_stub PASSED                                                                                     [ 62%]
tests/device_test.py::TestDeviceCommands::test_image_upstream_stub PASSED                                                                            [ 65%]
tests/device_test.py::TestDeviceCommands::test_reprovision_stub PASSED                                                                               [ 68%]
tests/device_test.py::TestDeviceCommands::test_ssh_stub PASSED                                                                                       [ 71%]
tests/device_test.py::TestDeviceCommands::test_update_stub PASSED                                                                                    [ 75%]
tests/image_builder_test.py::TestImageBuilder::test_build_all_stub PASSED                                                                            [ 78%]
tests/image_builder_test.py::TestImageBuilder::test_build_components_stub PASSED                                                                     [ 81%]
tests/image_builder_test.py::TestImageBuilder::test_builder_initialization PASSED                                                                    [ 84%]
tests/manifest_test.py::TestImageManifest::test_has_component PASSED                                                                                 [ 87%]
tests/manifest_test.py::TestImageManifest::test_load_manifest PASSED                                                                                 [ 90%]
tests/manifest_test.py::TestImageManifest::test_missing_components PASSED                                                                            [ 93%]
tests/manifest_test.py::TestImageManifest::test_resolve_path_relative PASSED                                                                         [ 96%]
tests/manifest_test.py::TestImageManifest::test_resolve_path_url PASSED                                                                              [100%]

==================================================================== 32 passed in 0.10s ====================================================================