tests: Set locale to C.UTF-8 in datetime tests

It was previously set (by default) to `C`, so this commit doesn’t change
the locale behaviour of the tests, but does ensure that messages printed
by the tests are correctly formatted in UTF-8 rather than transliterated
to ASCII.

That makes interpreting test output easier.

Signed-off-by: Philip Withnall <philip@tecnocode.co.uk>

glib/tests/gdatetime.c

@@ -3373,6 +3373,7 @@ main (gint   argc,
    * categories. Unset it to avoid interference with tests. */
   g_unsetenv ("CHARSET");
 
+  setlocale (LC_ALL, "C.UTF-8");
   g_test_init (&argc, &argv, NULL);
 
   /* GDateTime Tests */