Add scaffolding for a new test-internals binary

* There is a gcal_tests_add_internal() function...

* called from tests/test-internals.c...

* ... which doesn't add any tests yet, but we'll have some soon.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/550>

src/gui/event-editor/gcal-schedule-section.c

@@ -949,3 +949,9 @@ gcal_schedule_values_free (GcalScheduleValues *values)
 
   g_free (values);
 }
+
+void
+gcal_schedule_section_add_tests (void)
+{
+  /* g_test_add_func ("/event_editor/schedule_section/...", ...); */
+}