[lit, shtest-timeout] Always use an internal shell for the shtest-timeout to...

[lit, shtest-timeout] Always use an internal shell for the shtest-timeout to diagnose buildbot failures

Summary:
Right now this test is failing on the builtbots on Windows but we have a very similar setup where the test passes. The test is meant to test that specifying a timeout works correctly by running an infnite loop and having it timeout - on the buildbot, the infinite loop doesn't actually execute. This change runs all of the tests in the set using an internal shell rather than an external shell. I expect this will make the test pass which means that either the way the external shell is invoked or the external shell setup on the buildbots is not correct. Regardless of whether the test passes with this change, we'll need to undo this change and have a real fix.

@gkistanova was able to get logs from the buildbot to rule out a number of theories as to why this test is failing, but they didn't have enough information to confirm exactly what the issue is. The purpose of this change is to narrow it down, but if someone has a local repro and can aid in debugging, that would make it much speedier (and less prone to making the bots fail).

Reviewers: gkistanova, asmith, zturner, modocache, rnk, delcypher

Reviewed By: rnk

Subscribers: delcypher, llvm-commits, gkistanova

Differential Revision: https://reviews.llvm.org/D51326

llvm-svn: 340840