Infra: refactor global snap test

[liangmiQwQ]

At the moment, Vite+ runs global snap test with the real Vite+ installed on the contributor's computer, and real VP_HOME (~/.vite-plus). It means every test command which runs vp is nothing different from directly running vp in the terminal.

This can make problems:

1. Some commands' output will be influenced by contributors' environment. Like vp list -g, vp env list -g, vp env outdated -g.
2. Running some commands will modify contributors Vite+ setting and environment. Like vp install -g, vp env default. Even if it's cleaned after the test completed, there might still something left. (Like vp env default, we only set to lts)

Personally, I've already met some cases, where I want to use global snap test, but I can't considering contributors' computer environment #1659 (comment) #1698 (comment) #2003 (comment)

To improve that, I suggest to install Vite+ in another place for global snap tests, and use fake VP_HOME, and we will have two implementation method:

1. Use custom VP_HOME. It's direct but probably not stable (Just fixed some in fix(global): respect custom VP_HOME #2029, Vite+ still falls back to ~/.vite-plus for some cases)
2. Custom HOME. So Vite+ can still install into $HOME/.vite-plus.

@jong-kyung @fengmk2 Have you ever met similar situations? WDYT?

[fengmk2]

I have a snap test refactor task. The current plan is to follow the approach in https://github.com/voidzero-dev/vite-task/blob/main/crates/pty_terminal_test/README.md and support interactive CLI testing, then start migrating from new test cases and gradually move the old tests over.

The issue you mentioned can be handled after this refactor, so we won’t be blocked by the existing snap test setup. The current approach is too hard to change, and I don’t want to refactor it directly.