Continue having a tor_gmtime_impl() unit test so that we can detect
any problems in our replacement function; add a new test function to
make sure that gmtime<->timegm are a round-trip on now-ish times.

This is a fix for bug #40383, wherein we ran into trouble because
tor_timegm() does not believe that time_t should include a count of
leap seconds, but FreeBSD's gmtime believes that it should.  This
disagreement meant that for a certain amount of time each day,
instead of calculating the most recent midnight, our voting-schedule
functions would calculate the second-most-recent midnight, and lead
to an assertion failure.

I am calling this a bugfix on 0.2.0.3-alpha when we first started
calculating our voting schedule in this way.