tests: Skip the gdatetime month names test when running uninstalled
tests: Skip the gdatetime month names test when running uninstalled

It needs the translations to be installed. See the comment added to the
code.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=794180

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib