Add tests for PHPUnit\Util\ThrowableToStringMapper

[miladev95]

Adds a single, small unit test file tests/unit/Util/ThrowableToStringMapperTest.php that covers deterministic branches in PHPUnit\Util\ThrowableToStringMapper:
ErrorException -> message only (exact)
Generic Throwable -> ClassName: message\n (exact)
ExpectationFailedException with ComparisonFailure -> message + diff + newline (substring checks)
PhptAssertionFailedError -> message + diff + newline (substring checks)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.86%. Comparing base (cb8b1ac) to head (3367e02).
⚠️ Report is 17 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #6448   +/-   ##
=========================================
  Coverage     95.86%   95.86%           
  Complexity     7295     7295           
=========================================
  Files           792      792           
  Lines         22442    22442           
=========================================
  Hits          21514    21514           
  Misses          928      928           

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[sebastianbergmann]

Cherry-picked into 10.5 and merged to 11.5, 12.5, and main from there.