Fix a bit of libc++-specific behavior in the regex tests; add a missing test....

Fix a bit of libc++-specific behavior in the regex tests; add a missing test. Reviewed as https://reviews.llvm.org/D57391 Thanks to Andrey Maksimov for the patch

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@352781 91177308-0d34-0410-b5e6-96231b3b80d8