tests: fix check-qom-interface leaks

Found thanks to ASAN.
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>

tests/check-qom-interface.c

@@ -76,6 +76,7 @@ static void test_interface_impl(const char *type)
 
     g_assert(iobj);
     g_assert(ioc->test == PATTERN);
+    object_unref(obj);
 }
 
 static void interface_direct_test(void)