tests: Set both environment variable and locale when running tests

Non-glibc gettext implementation seems to decide the language from
LC_MESSAGES environment variable instead of LC_MESSAGES locale, so
we should set both environment variable and locale when running tests
which need translation from specific languages.

gio/tests/gapplication.c

@@ -972,6 +972,8 @@ test_api (void)
 int
 main (int argc, char **argv)
 {
+  g_setenv ("LC_ALL", "C", TRUE);
+
   g_test_init (&argc, &argv, NULL);
 
   g_test_dbus_unset ();