prefork model with warm test dependencies

instead of shelling to ruby for railties tests, we'll use a fork to isolate any weird issues. In addition, load up `abstract unit` to warm up the environment, so each test can skip over the env booting.

# TODO

- [ ] preserve exit code on failure
- [ ] print which file is being run before loading for debugging
- [ ] load more dependencies (all the helpers?) ahead of time. Maybe even make a generalized helper for this and load that file in the prefork for simplicity
- [ ] some files are reporting 0 runs, so maybe not all tests are actually being run?