[lit] Disable shtest-timeout on Windows

Summary: This is the only test that is still failing on Windows - or rather, it is expected to fail on the bots, but passes on the new bot that we're preparing causing a failure, so I'm going to disable it. Since the test has rarely, if ever, passed on the bots, this should have the same effect and it will unblock the creation of the new bot.

Reviewers: asmith, delcypher, zturner

Subscribers: stella.stamenova, llvm-commits

Differential Revision: https://reviews.llvm.org/D51871

llvm-svn: 341856