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>

tests/test-internals.c

@@ -0,0 +1,17 @@
+#include <glib.h>
+#include <locale.h>
+
+#include "gui/gcal-tests.h"
+
+int
+main (int argc, char **argv)
+{
+  g_setenv ("TZ", "UTC", TRUE);
+
+  g_test_init (&argc, &argv, NULL);
+  g_test_bug_base ("https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/");
+
+  gcal_tests_add_internals ();
+
+  return g_test_run ();
+}