[Lit] Fix flaky test on heavily loaded bots
On some of the slow or heavily-loaded bots, this test was failing
intermittently because the infinite_loop.py script might not emit
anything to stdout before the 1 second timeout, so the "Command Output"
line isn't present in the output. That output isn't really important to
this test, we just care that the process is killed, so we can just rmove
that check line from the test.
Differential revision: https://reviews.llvm.org/D92563