Guard a number of tests relying on threads support when built in
single-threaded mode.

Differential Revision: http://reviews.llvm.org/D14731



git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@264191 91177308-0d34-0410-b5e6-96231b3b80d8
15 files changed