dircolors: turn integration test into unit tests

[cakebaker]

This PR turns an integration test into three unit tests, allowing us to make the StrUtils trait private.

There is another integration test that isn't an integration test. To keep this PR small, I left it for a future PR as it also should integrate changes from #12228

[oech3]

I don't see any confliction with the linked PR. Am I missing something?

[cakebaker]

@oech3 there isn't a conflict with the linked PR. I linked to it because it modifies test_shell_syntax() which is the other integration test that should be a unit test.

And while I think your motivation in the linked PR is to remove the unsafe code, there is also a testing aspect to your changes, and thus it probably makes sense to combine the changes.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/tail/truncate. tests/tail/truncate is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/tail-n0f (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/resolution (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/seq/seq-epipe is now being skipped but was previously passing.

[oech3]

I don't understand how you want to combile it currently. Would you cherry-pick the PR when it happens and close it?

[cakebaker]

Changes since last push:

• rebased and fixed conflicts in both files