tests: Continue closure-refcount test until all three threads are seen

Previously, the test assumed that thread1 and thread2 would be scheduled
enough to set seen_thread{1,2} by the fact that the test runs for a high
number of iterations. On some platforms/schedulers, that’s not true,
which causes the test to spuriously fail.

Fix that by forcing the test to continue iterating until both threads
are seen. If this takes too long, the Meson test runner timeout will be
hit and the test will be terminated.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
7 jobs for refcount-fixups in 16 minutes and 23 seconds (queued for 2 seconds)