tests/gem_flink_race: actually make it somewhat useful

tests/gem_flink_race: actually make it somewhat useful

- Enable subtest support.

- Add a check for the same flink name in the racing threads, which is
  an issue one of my recent patches actually fixes.

- Add the test I've actually wanted to write which races an flink
  against gem close (with no open in between). That one does indeed
  leak.

- Readd the leak check, but note that this needs a fixed kernel.
  Otherwise the leak counter will be utter garbage.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>