GDateTime test: fix occasional failures

We were using the time() library call to get the current time from the
system in order to compare it to the time returned by
g_date_time_new_now().

Of course, we took care to ensure that the time (in seconds) didn't
change in the middle of this process by checking the before and after
value of the system time.

Unfortunately, the system time as measured by time() was being taken
from a less-accurate clock source than the time used by GDateTime.  As a
result, we could have GDateTime already into the next second while the
"seconds" value of the time returned by time() was still in the last
one, even when checked "after".

Avoid the problem by using the same ultimate source for time --
g_get_real_time().

This is based on a similar patch from Iain Lane, but it uses
g_get_real_time() instead of g_get_current_time().

https://bugzilla.gnome.org/show_bug.cgi?id=754994