tests: Add copyright/licensing header to assert-msg-test

This is put together through git archaeology:
```
git log -- glib/tests/assert-msg-test.c tests/assert-msg-test.c
```

The following commits were too trivial to have meaningful copyright:
 - 8e59d8602c
 - 44c004c84e
 - 207b8cb8a5
 - a1bee97d4f

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Helps: #1415

glib/tests/assert-msg-test.c

@@ -1,3 +1,23 @@
+/* GLib testing framework examples and tests
+ *
+ * Copyright © 2009 Canonical Limited
+ *
+ * SPDX-License-Identifier: LGPL-2.1-or-later
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library 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
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General
+ * Public License along with this library; if not, see <http://www.gnu.org/licenses/>.
+ */
+
 #include <glib.h>
 
 int