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.
Example: suppose I have some arbitrary management diktat saying every machine must reboot at least once per season; I can write policy to discover my boot time, with any luck, and then condition a processes or command promise, to do the shut down, on a test that we're 91 days, or three months, later than that.  It'll look a bit like the example I gave above, using sum().

Now consider a case where I've been crazy enough to write a CI system *in* cfengine (somehow) and the part of it that runs tests supports each test coming with a wait time (specified in minutes), after which it's to be shot down and reported as failing because it was taking longer than it should (to prevent the whole system from grinding to a halt thanks to a hang).  My "kill that test now" promise is going to want to use a computed laterthan() which adds the timeout to the test's start-time.  Of course, this is a CI system, so it's going to run at weird times of day, week, month and year (because that's when developers commit fixes to the bugs their customers desperately want fixed by yesterday), so my arithmetic (if you demand that I not use denormal times) is going to need to handle over-flow from minutes into hours, from hours into days, from days into months (and that's one insanely hairy one to do by any standard - don't forget leap years) and from months into years.  I don't even know how to start with the simple "reduce modulo 60" part in the first step of that; do you really want to demand that anyone writing policy to do something non-trivial should also solve this problem ?

Writing policy that has a hard-coded absolutely fixed date in it, that only changes because the admin has updated the policy with a new hard-coded date (which seems to be the use-case you both have in mind), strikes me as unlikely to happen much at all, much less be *more* common that uses that involve arithmetic and depend on sensible handling of denormal dates and times.