Remove DateTime from tests

[m-dango]

The only remaining non-core dependency on the track, aside from Test2 related modules, is DateTime. I suspect that it shouldn't be necessary to keep, and it would be good to remove in the process of updating exercises in #249.

[sshine]

Exercises that still use DateTime:

• meetup (Generate meetup with Test2::V0 #309)
• gigasecond (Generate gigasecond with Test2::V0 #308)