regex-tests: skip UTF-8 test on mingw test-regex failed on mingw; and I traced it in gdb to an instance of init_dfa() setting dfa->is_utf8 to 0 in spite of setlocale() claiming success for "en_US.UTF-8". test-wcwidth already has precedent for skipping utf-8 tests where the system (or gnulib setlocale replacement, in this case) lies about utf-8 support. * modules/regex-tests (Depends-on): Add localcharset. * tests/test-regex.c (main): Use it to skip test on mingw. Signed-off-by: Eric Blake <eblake@redhat.com>