tests: add missing call to g_type_init()

This patches fixes the following error when testing ModemManager with
glib < 2.36, which still requires g_type_init() to be called:

TEST: test-service-generic... (pid=20166)
  /MM/Service/Generic:
(/work/ModemManager/plugins/.libs/lt-test-service-generic:20166): GLib-GObject-CRITICAL **: gtype.c:2720: You forgot to call g_type_init()
FAIL

plugins/generic/tests/test-service-generic.c

@@ -76,6 +76,7 @@ test_something (TestFixture *fixture)
 int main (int   argc,
           char *argv[])
 {
+    g_type_init ();
     g_test_init (&argc, &argv, NULL);
 
     TEST_ADD ("/MM/Service/Generic", test_something);