Found six (nmostly) identical files named 'test_allocator.h' in the libcxx...

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.

llvm-svn: 196174