systemc: Teach verify.py how to expect failing error codes.

systemc: Teach verify.py how to expect failing error codes.

Some tests expect to fail. For those tests (and only those tests) we
need to tell verify.py that it's ok if their exit status isn't 0. Also
if those tests *don't* fail, then that will also be flagged as an
error.

This is done by adding an expected_returncode file into the test's
source directory which holds what the expected return code should be.

Change-Id: I239a28e1d98dd3f76b71028660e492f675a0b3cb
Reviewed-on: https://gem5-review.googlesource.com/c/12446
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>