systemc: Teach verify.py to diff files when checking test results.

systemc: Teach verify.py to diff files when checking test results.

Currently it just diffs the stdout and ignores other reference files.
It also doesn't filter out noise in the diffs from non test related
simulator messages. These include startup messages, messages when the
simulator finishes executing, and some non-standard warnings, etc.

Change-Id: Idcb19edd893cd8818423c2c5ebb6cbfb278baffa
Reviewed-on: https://gem5-review.googlesource.com/12054
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>