tests: fix shm test deadlock

The test was not bringing the source back to NULL, so its thread
could start creating a buffer while the pads were being shutdown.

tests/check/elements/shm.c

@@ -72,6 +72,8 @@ setup_shm (void)
 static void
 teardown_shm (void)
 {
+  fail_unless (gst_element_set_state (src, GST_STATE_NULL) ==
+      GST_STATE_CHANGE_SUCCESS);
   gst_check_teardown_sink_pad (src);
   gst_check_teardown_src_pad (sink);
   gst_check_teardown_element (src);