[libcxx] Remove use of uniform initialization from regex tests so that they compile in C++03.

Reviewers: danalbert, jroelofs, mclow.lists

Reviewed By: mclow.lists

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D5957

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@220707 91177308-0d34-0410-b5e6-96231b3b80d8