[TSan][libdispatch] Replace NSTemporaryDirectory in tests

After this change, most tests don't have a dependency on Foundation.

Note: To hold the file name `tempnam` allocates a new buffer. We leak
      this buffer (omit the free), but I don't think we need to care.

Reviewed By: kubamracek

Differential Revision: https://reviews.llvm.org/D60591

llvm-svn: 358308