porting findutils testing to use uutests

[sarnobat]

Trying to pick up the dormant work of:
#541
Original issue:
#537

Example using new library:

• https://github.com/uutils/coreutils/blob/main/tests/by-util/test_ls.rs
• https://github.com/uutils/coreutils/blob/main/tests/by-util/test_cp.rs

[sarnobat]

@sylvestre - newbie question:

• Is it possible to get some feedback on the code changes in this PR before I replicate the same effort for more tests? (I'm also wondering what workflow failures might be invisible to me since it requires maintainer approval)
• Must all the changes be done in a single pull request? Or can it be done in small changes? (I prefer the latter but understand that that may be unrealistic)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.88%. Comparing base (69b276a) to head (c98f65b).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #578   +/-   ##
=======================================
  Coverage   88.88%   88.88%           
=======================================
  Files          31       31           
  Lines        6300     6300           
  Branches      324      324           
=======================================
  Hits         5600     5600           
  Misses        578      578           
  Partials      122      122           

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[sylvestre]

That looks great! And no worries for doing it in the same pr