Fixed race conditions in thread tests; exposed by UBSan

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@178029 91177308-0d34-0410-b5e6-96231b3b80d8
More vector debug tests.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@178033 91177308-0d34-0410-b5e6-96231b3b80d8