Add -q flag to runtests.sh to suppress diff and update Usage note

No diff / quiet mode is useful to run & commit tests on master followed by
running & diffing on a patch set in a single commandline.

Change-Id: If0c52e441ed6fec9f4c4937b18bfebd4519000a8
Merge "Add -q flag to runtests.sh to suppress diff and update Usage note"