Autotest: Fix QDateTime failure if the test got run West of Greenwich

There was a comment about the test failing for max() because of an
overflow. That happens if you're at UTC or ahead of it (to the East of
the Prime Meridian), which is how this test usually gets run (UTC,
Europe/Oslo, Europe/Helsinki, Pacific/Auckland). But if you're behind
UTC (to the West of the Prime Meridian), then the overflow happens for
min().

Change-Id: Iebba49d1303e9f18f5038f5cf23c77bf83e5fd4b
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>