Quality assurance: add/perform domain-equivalence testing

[jayaddison]

Iterate through the range of possible datetime.timedelta values, and for each one (or a subset?), ensure that a corresponding and comparatively-equal timedelta_isoformat.timedelta object can be constructed and round-tripped from object-to-string-and-back.

This could be computationally expensive, depending on how it's achieved - so I don't know whether it should be added to the tests directory.