Generate split test data via split

@cpennington I finally did what you asked me to do many months ago (was necessary step before changing repr of blocks which I didn't get to.) Please review.

@singingwolfboy please be 2nd reviewer

Note, I removed the date checking b/c datestamps in mongo seem slightly out of sync w/ dates in python and the test code no longer controls the datestamps.

I removed the pretty id check b/c it failed but I have another PR getting rid of pretty ids; so, I didn't want to figure out the cause of the failure.