tests: Added functionality to allow the ignoring of test suites

tests: Added functionality to allow the ignoring of test suites

Previously, when `tests/main.py run` was executed all the tests found
were run. It is now necessary to ignore some test suites as they fail.
Therefore, `gem5/suite.py` has been updated to read from `gem5/.testignore`
(if present). This file contains a list of all the test suites which are to
be ignored.

Change-Id: I699ea662b701d82199980084261496f24b13d340
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23023
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>