tests: Only run Ruby tests when testing Ruby targets

tests: Only run Ruby tests when testing Ruby targets

Limit the test configs to Ruby-only configs when testing a Ruby target
that isn't MI_example. This avoids re-running configs that has already
been tested by the generic (non-Ruby) ISA target. This behavior was
the expected behavior prior to switching to the new test framework.

Change-Id: I3f138dbf9c7071ce862d1073aaec57c59afbc921
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>