laterthan fix and acceptance tests

Current state makes it useless.

Redmine: https://dev.cfengine.com/issues/6530

Please backport to 3.6.x for inclusion in 3.6.2.
For that matter, I'd be in favour of allowing negative values, for similar reasons; laterthan(2015,0,0,0,0,-1) is so much easier to read than laterthan(2014,11,30,23,59).  Consider, also, the semantic difference between laterthan(2015,0,-7,0,0,0), the last week of the year and onwards, vs laterthan(2014,11,24,0,0,0) which appears to care about the day in question being Christmas eve, where actually I just wanted to specify the last week of the year and my reasons had nothing to do with Christmas, aside from accidentally coinciding with it.