[lit] Skip stop-hook test on Windows

This test is now marked as unsupported on Windows - it is not technically  "unsupported" on Windows, but it fails because "expr ptr" does not evaluate correctly. However, the error message contains the expected string, so the test "passes" despite the fact that the commands failed
The following bug has been opened for it: llvm.org/pr40119

llvm-svn: 349784