drm/i915/selftests: Flush live_evict

Be sure to cleanup after live_evict by flushing any residual state off
the GPU using igt_flush_test.

Tvrtko mentioned that it is probably wise to stop repeating this ad hoc
around the tests and implement a live test runner.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190618161951.28820-1-chris@chris-wilson.co.uk