tests: Make testboxes an actual test

testboxes was a binary that did unit testing, but it wasn't integrated
to the test system, so in effect, it was never run. Instead integrate it
into the other mutter unit tests. This includes changing a few of
meta_warning()s into g_warning()s so that the GTest framework can handle
them.

src/tests/boxes-tests.h

@@ -0,0 +1,23 @@
+/*
+ * Copyright (C) 2018 Red Hat, Inc.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef BOXES_TESTS_H
+#define BOXES_TESTS_H
+
+void init_boxes_tests (void);
+
+#endif /* BOXES_TESTS_H */

src/tests/unit-tests.c

@@ -28,6 +28,7 @@
 #include "compositor/meta-plugin-manager.h"
 #include "core/boxes-private.h"
 #include "core/main-private.h"
+#include "tests/boxes-tests.h"
 #include "tests/meta-backend-test.h"
 #include "tests/monitor-config-migration-unit-tests.h"
 #include "tests/monitor-unit-tests.h"
@@ -252,6 +253,7 @@ init_tests (int argc, char **argv)
   init_monitor_store_tests ();
   init_monitor_config_migration_tests ();
   init_monitor_tests ();
+  init_boxes_tests ();
 }
 
 int