tests,udev-rules: add dummy test to avoid -Wunused-function warnings

If none of the plugins enabled in the build has custom udev rules, the
common_test() method would be unused. Avoid this just by adding a new
dummy test which is always available in the udev rules tester.

plugins/tests/test-udev-rules.c

@@ -35,6 +35,9 @@ common_test (const gchar *plugindir)
     GArray *rules;
     GError *error = NULL;
 
+    if (!plugindir)
+        return;
+
     rules = mm_kernel_device_generic_rules_load (plugindir, &error);
     g_assert_no_error (error);
     g_assert (rules);
@@ -43,6 +46,14 @@ common_test (const gchar *plugindir)
     g_array_unref (rules);
 }
 
+/* Dummy test to avoid compiler warning about common_test() being unused
+ * when none of the plugins enabled in build have custom udev rules. */
+static void
+test_dummy (void)
+{
+    common_test (NULL);
+}
+
 /************************************************************/
 
 #if defined ENABLE_PLUGIN_HUAWEI
@@ -176,6 +187,7 @@ int main (int argc, char **argv)
     setlocale (LC_ALL, "");
 
     g_test_init (&argc, &argv, NULL);
+    g_test_add_func ("/MM/test-udev-rules/dummy", test_dummy);
 
 #if defined ENABLE_PLUGIN_HUAWEI
     g_test_add_func ("/MM/test-udev-rules/huawei", test_huawei);