Parallelize src/test/test into chunks.

[nmathewson]

First, we introduce a flag to teach src/test/test to split its work into chunks. Then we replace our invocation of src/test/test in our "make check" target with a set of 8 scripts that invoke the first 8th of the tests, the second 8th, and so on.

This change makes our "make -kj4 check" target in our hardened gitlab build more than twice as fast, since src/test/test was taking the longest to finish.

Closes 40098.