test/librbd: allow parallel runs of run-rbd-unit-tests

test/librbd: allow parallel runs of run-rbd-unit-tests

Running all tests sequential makes it the longest test of
`make check`, with each partial test taking around 500 sec.
Running 6 tests thus takes almost an hour.

Cut this down if ctest runs tests in parallel

Default behaviour of src/test/run-rbd-unit-tests.sh is kept:
  Without parameters the tests are run in sequence

To run unitttest_librbd with RBD_FEATURES, use `N` as parameter

Signed-off-by: Willem Jan Withagen <wjw@digiware.nl>
(cherry picked from commit e2f292983852f486c1eb56f0f83f8f29d47b96dd)