testutils: Run in UTF8 locale

before, code was using the "C" locale, but that one uses ASCII. Instead,
run in the "C.utf8" locale.

Nobody expects code to not support UTF8 and no end user runs their
machine in an ASCII setup, so it makes no sense to default to that.

gtk/gtktestutils.c

@@ -77,7 +77,7 @@ gtk_test_init (int    *argcp,
 {
   g_test_init (argcp, argvp, NULL);
   gtk_disable_setlocale();
-  setlocale (LC_ALL, "C");
+  setlocale (LC_ALL, "en_US.UTF-8");
   g_test_bug_base ("http://bugzilla.gnome.org/show_bug.cgi?id=%s");
 
   /* XSendEvent() doesn't work yet on XI2 events.