t4210: detect REG_ILLSEQ dynamically and skip affected tests

t4210: detect REG_ILLSEQ dynamically and skip affected tests

7187c7bbb8 (t4210: skip i18n tests that don't work on FreeBSD, 2019-11-27)
adds a REG_ILLSEQ prerequisite, and to do that copies the common branch in
test-lib and expands it to include it in a special case for FreeBSD.

Instead; test for it using a previously added extension to test-tool and
use that, together with a function that identifies when regcomp/regexec
will be called with broken patterns to avoid any test that would otherwise
rely on undefined behaviour.

The description of the first test which wasn't accurate has been corrected,
and the test rearranged for clarity, including a helper function that avoids
overly long lines.

Only the affected engines will have their tests suppressed, also including
"fixed" if the PCRE optimization that uses LIBPCRE2 since b65abcafc7
(grep: use PCRE v2 for optimized fixed-string search, 2019-07-01) is not
available.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>