Run tests in parallel

[jfecher]

Currently all tests are tested sequentially. The inputs of each test do not depend on any other test so we could run them in parallel batches instead to improve testing speed particularly when there are many test files.

[osa1]

I think another useful feature would be the ability to run a single test file.

Once that's implemented, this feature can be emulated using something like xargs -P or parallel, though the output may not be as good as implementing the support in golden-tests.