[CMake] Disable adding the test suite as a projects subdirectory

[CMake] Disable adding the test suite as a projects subdirectory

This will never work as an add_subdirectory call, so we should just make sure it doesn't happen. To do this properly we'll need to add it under clang similar to the external compiler-rt.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@251543 91177308-0d34-0410-b5e6-96231b3b80d8