igt/gem_sync: Enforce a timeout of 20s

The sync test is supposed to complete in 10s. But some bugs cause it to
run very, very slowly. As a defence against those, terminate the test if
we wait for more than 20s.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

tests/gem_sync.c

@@ -155,7 +155,7 @@ sync_ring(int fd, unsigned ring, int num_children)
 
 		gem_close(fd, object.handle);
 	}
-	igt_waitchildren();
+	igt_waitchildren_timeout(20, NULL);
 	igt_assert_eq(intel_detect_and_clear_missed_interrupts(fd), 0);
 }