[Lit] Fix flaky test on heavily loaded bots

[ostannard]

Summary

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 remove that check line from the test.

[rovka]

Thanks for looking into this, Oliver! This seems sensible to me, but I think @delcypher should probably have the final say.

[dblaikie]

Makes sense to me (I can verify the failure mode by adding a sleep inside infinite_loop.py before it prints anything - and that this change fixes the issue) and this seems to be causing ongoing pain, so let's get this committed & @delcypher can weigh in post-commit when they have a chance.

[delcypher]

LGTM. @ostannard Thanks for investigating!

llvm/utils/lit/tests/shtest-timeout.py

 # RUN: --param set_timeout=1 > %t.cfgset.out 2> %t.cfgset.err
 # RUN: FileCheck --check-prefix=CHECK-OUT-COMMON  < %t.cfgset.out %s
 # RUN: FileCheck --check-prefix=CHECK-CFGSET-ERR < %t.cfgset.err %s
 #
 # CHECK-CFGSET-ERR: Using internal shell
 
 # CHECK-OUT-COMMON: TIMEOUT: per_test_timeout :: infinite_loop.py
 # CHECK-OUT-COMMON: Timeout: Reached timeout of 1 seconds
-# CHECK-OUT-COMMON: Command {{([0-9]+ )?}}Output
 # CHECK-OUT-COMMON: Timed Out: 1
 
 
 ###############################################################################
 # Check tests can complete in with a timeout set
 #
 # `short.py` should execute quickly so we shouldn't wait anywhere near the
 # 3600 second timeout.