test framework: ignore sighups

On OS X, we're occasionally seeing sighups come in to what
looks like the whole test runner process group (all the
multiprocessing workers and the main process).  It's not due
to a lost console.  This change has the main parallel test runner
process and the child worker processes ignore sighup.

Covers:
https://llvm.org/bugs/show_bug.cgi?id=24846

llvm-svn: 248141