Fix shtest-timeout test harder

Don't produce or expect any output from the infinite looping test -
doing so is a recipe for racey flakyness without a longer timeout to
ensure the output is received first, even though that doesn't seem
integral/important to the test. Instead have a plain, no output infinite
loop and check that that is caught and handled.

If for some reason the output is valuable for test coverage - the
timeout should be increased from 1 second to give the process time to
output the text, flush, and for that text to be received and buffered
before the test is timed out.

Author: dwblaikie

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

@@ -1,9 +1,3 @@
 # RUN: %{python} %s
-from __future__ import print_function
-
-import sys
-
-print("Running infinite loop")
-sys.stdout.flush() # Make sure the print gets flushed so it appears in lit output.
 while True:
     pass

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

@@ -25,7 +25,6 @@
 # RUN: FileCheck --check-prefix=CHECK-INTSH-ERR < %t.intsh.err %s
 
 # CHECK-INTSH-OUT: TIMEOUT: per_test_timeout :: infinite_loop.py
-# CHECK-INTSH-OUT: command output:
 # CHECK-INTSH-OUT: command reached timeout: True
 
 # CHECK-INTSH-ERR: Using internal shell