Found six (nmostly) identical files named 'test_allocator.h' in the libcxx test suite. Moved one to /support, made it a superset, and removed all but one of the others, and iupdated all the includes. Left the odd one (thread/futures/test_allocator.h) for later.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@196174 91177308-0d34-0410-b5e6-96231b3b80d8