find: use GNU wording "unknown predicate" for unrecognized args

[sylvestre]

GNU find rejects an unrecognized predicate with
"find: unknown predicate ''", but we printed
"find: Unrecognized flag: ''". Match GNU so the tests/find/refuse-noop GNU test (and others hitting the same path) pass. Add an integration test covering -noop and ---noop.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.91%. Comparing base (5398003) to head (efe6dc0).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #724      +/-   ##
==========================================
+ Coverage   91.85%   91.91%   +0.05%     
==========================================
  Files          35       35              
  Lines        7149     7202      +53     
  Branches      375      380       +5     
==========================================
+ Hits         6567     6620      +53     
  Misses        437      437              
  Partials      145      145              

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[codspeed-hq]

Merging this PR will not alter performance

✅ 20 untouched benchmarks

---

_{Comparing find-unknown-predicate-message (efe6dc0) with main (001966f)}