[lit] Don't run `slow.py` in `shtest-timeout.py` test.

The program used to be used in `quick_then_slow.py` but that was
removed in r328702. The tests always run `slow.py` on its own but
this doesn't really test additional code so we'll just drop running
`slow.py` so the tests run faster.

rdar://problem/40221572

llvm-svn: 332986