Short follow-up to #5380 to support escapes for closing brackets in POSIX regexes in accordance with the most recent POSIX standard version: https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1_chap09.html#tag_09_03_02 for basic and https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1_chap09.html#tag_09_04_02 for extended regexes.

Because of the (expected) test failures caused by this change, I noticed that the tests in gh_5244_atomescape_posix_common are called twice for extended and egrep from test_gh_5244_atomescape_extended_or_egrep: Once via gh_5244_atomescape_posix_not_awk and once via gh_5244_atomescape_extended_egrep_awk. That's once more than necessary, so let's get rid of this double-testing. I opted to remove the call of gh_5244_atomescape_posix_common in gh_5244_atomescape_extended_egrep_awk, but this means a call has to be added to gh_5244_atomescape_awk so that these common POSIX tests are still exercised in awk mode.