Introduce clean-ups in llvm/unittests/Makefile.unittest, to sweep stray *Tests.

Stray *Tests might stay after reverting.

FIXME: Could we apply this feature to clang/unittests?
FIXME: Implement this feature to CMake.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197661 91177308-0d34-0410-b5e6-96231b3b80d8