Multithreaded test suite

[staticfloat]

I've noticed that this package is sucking up a lot of CI time. I will never argue for running fewer tests as I love good test coverage, and I understand that numerical projects such as these just can't use toy problems that are quick to solve to give good test coverage.

Examining an exemplary test run, I see that this run took ~20hs of CI time, the majority of which is in NeuralAdapter. Logging into the test machines, I noticed that all the Julia processes were pretty much single-threaded. Is there a way to split the tests into separate pieces that can be run in parallel on a single machine? If not, fine, we live with it.

[sathvikbhagavan]

Now, we have all the test suites to run under one hour. @ChrisRackauckas, can we close this?