dircolors: turn integration test into unit test & remove unsafe code

[cakebaker]

This PR does the following changes:

• turns an integration test into an unit test
• makes a function and an enum private
• removes unsafe code in the unit test (changes are adapted from dircolors: remove unsafe #12228 and so I added @oech3 and @xtqqczze as co-authors)
• removes the OutputFmt::Unknown variant

Closes #12228

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/head/head-write-error is no longer failing!
Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.
Congrats! The gnu test tests/printf/printf-surprise is now passing!
Skip an intermittent issue tests/cut/bounded-memory (was skipped on 'main', now failing)

[oech3]

Can we avoid unwrap(), unwrap(),... and unwrap() caused by Some(None)?

[cakebaker]

Can we avoid unwrap(), unwrap(),... and unwrap() caused by Some(None)?

I'm not sure I understand you correctly: do you suggest that I should use Some(OutputFmt::<some variant>) instead of using unwrap()s in the test?

Update: I removed the unwrap()s in the latest push.

[oech3]

Thankyou. We still have many Some(), but still better than unwrap().