drm/i915/selftests: Teach hang-self to target only itself

We have a test case to exercise resetting an engine while the other
engines are busy, all the TEST_SELF adds on top is that the target
engine also has background activity. In this case it is useful to first
test resetting the engine while there is background activity, as a
separate flag from exercising all others.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200607222108.14401-3-chris@chris-wilson.co.uk