Parallelize several tests to make hardened-build CI faster.

Right now our hardened-build gitlab CI test is much slower than the others. It looks like when the build is hardened, ./src/test/test is slowest.

As a solution, I think we should have the ability to launch the first 1/N of the tests in src/test/test in parallel with the second 1/N, the third 1/N, and so on.

I'm running an experiment to see if this helps.