glib/tests/gdatetime: use UTC time in test_GDateTime_diff()

test_GDateTime_diff() checks that the span from 2009-01-01 to
2010-01-01 is exactly 365 * G_TIME_SPAN_DAY, but it does this using
local time, and so fails if you are in a timezone that is in the
southern hemisphere which only did DST during one of 2008-2009 and
2009-2010 (in which case the year will end up being one hour too long
or too short).

Switch the diff tests to use UTC time instead; there are plenty of
other local time tests already.

https://bugzilla.gnome.org/show_bug.cgi?id=701529