Merge branch 'cb/t4210-illseq-auto-detect'

Merge branch 'cb/t4210-illseq-auto-detect'

As FreeBSD is not the only platform whose regexp library reports
a REG_ILLSEQ error when fed invalid UTF-8, add logic to detect that
automatically and skip the affected tests.

* cb/t4210-illseq-auto-detect:
  t4210: detect REG_ILLSEQ dynamically and skip affected tests
  t/helper: teach test-regex to report pattern errors (like REG_ILLSEQ)