tests/gem_exec_reuse: Don't leak the hang detector

My new "are there any child processes left?" check in igt_exit catched
this one.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

tests/i915/gem_exec_reuse.c

@@ -213,4 +213,7 @@ igt_main
 		for (n = 0; n < ncontexts; n++)
 			gem_context_destroy(no.fd, contexts[n]);
 	}
+
+	igt_fixture
+		igt_stop_hang_detector();
 }