[TSan][libdispatch] Move libdispatch tests out of Darwin folder

The libdispatch tests are now not tied to Darwin anymore. Move them to
their own platform-independent folder.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@358316 91177308-0d34-0410-b5e6-96231b3b80d8
23 files changed