gdate: Comment that g_date_valid_dmy() does all necessary bounds checks

Make it more obvious that an explicit check isn’t needed for the upper
bound on years, since it’s limited by the type width.

Add a unit test to demonstrate this.

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

https://bugzilla.gnome.org/show_bug.cgi?id=540013