regex-tests: skip UTF-8 test on mingw

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>