Resolve test flake in flags_test

[evankanderson]

Summary

Don flagged this issue the other day. The simplest fix is to not run the sub-tests in parallel; I've also filed a comment on the upstream that it would be nice to be able to avoid the singleton, in the way that the net/http library has something like a "default ServeMux", but you can also create your own.

Change Type

Mark the type of change your PR introduces:

• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

Fix in tests, the tests are the test.

Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
• Checked that related changes are merged.

[stacklokbot]

✅ No Invisible Unicode Characters Detected.

[coveralls]

coverage: 49.294%. remained the same
when pulling 80dae64 on evankanderson:fix-flake
into 4c3fecd on stacklok:main.