uutests: gate by target support

[xtqqczze]

Blocked on #14242.

[github-actions]

Binary size comparison:

Individual binary size comparison VS main (threshold: >=5% AND >=4 KB).

Total size of compared binaries: 152.18 MB (+1000 KB, +0.65%)

Significant per-binary changes:
  comm     1.12 MB ->    2.33 MB  (+1.21 MB, +107.29%)

[codspeed-hq]

Merging this PR will not alter performance

✅ 3 untouched benchmarks
⏩ 409 skipped benchmarks¹

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_{Comparing xtqqczze:targets-uutests (a620f78) with main (bd240c0)²}

Footnotes

1. 409 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on main (e790081) during the generation of this report, so bd240c0 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[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/rm/isatty (passes in this run but fails in the 'main' branch)