Disable schema test if xmllint is too new

[scpeters]

🦟 Bug fix

Mitigation for #1656, manual backport of #1658 and part of #1655

Summary

As noted in #1656, INTEGRATION_schema_test fails with versions of xmllint newer than 2.15. This disables the test if the xmllint version is too new. It also fixes the warning when xmllint is not found.

Backport Policy

• This is safe to backport to the following versions:
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Checklist

• Signed all commits for DCO
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• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
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• All tests passed (See test coverage)
• Updated Bazel files (if adding new files). Created an issue otherwise.
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