tests: add ruby_mem_test.py to the tests

tests: add ruby_mem_test.py to the tests

This catches ruby functional memory errors we have observed, and ensures
that ruby_mem_test.py itself won't be broken.

The test duration is about 10 seconds, and it can be run as:

./main.py run --uid SuiteUID:tests/gem5/test_ruby_mem_test.py:test-ruby\
_mem_test-NULL-x86_64-opt

Change-Id: I39bc559aaea3ebb41217a96cd4e8dae46271ea1f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26805
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>