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/gcal-tests.c

@@ -0,0 +1,15 @@
+#include <glib.h>
+
+#include "gcal-tests.h"
+#include "event-editor/gcal-schedule-section.h"
+
+/* Adds all the tests for the internals of the GUI.
+ *
+ * To avoid exporting lots of little test functions from each source file, add a single
+ * gcal_foo_add_tests() that in turn adds unit tests for that file.
+ */
+void
+gcal_tests_add_internals (void)
+{
+  gcal_schedule_section_add_tests ();
+}