avoid splitting repo tests into too small parts around -J16

avoid splitting repo tests into too small parts around -J16

The initTests have to be run once per part, and a point of diminishing
returns can be reached where more work is being done to set up for 1 or
2 tests than to run them.

This is better than a hard cap of -J8 or so, because it lets other
things than these particular tests still be parallelized at -J16.

Sponsored-by: Dartmouth College's Datalad project