Autotest: Fix unexpected rounding of floating point in date tests

QString("new Date(%0)").arg(1380318500 * 1000.0) expands to "new
Date(1.38032e+12)". That's an unnecessary rounding and cuts off
interesting bits out of the date.

It took me quite a while debugging QScriptEngine trying to find out
why the time_t and the JS Date internal numbers were different.

Change-Id: Id880f3f11edd0af07e1726eac0242d4a9625c368
Reviewed-by: John Layt <jlayt@kde.org>