Fix and clean-up bad EAVT parsing spec.

This tests what I think the test was getting at in a more direct way.

/cc @charleso @raronson for review.
Should we still be testing for bad date/time in a particular zone? see https://github.com/ambiata/ivory/blob/master/ivory-core/src/test/scala/com/ambiata/ivory/core/Arbitraries.scala#L66-L81