Run tests in parallel (as an option)

[biffen]

It should be possible to supply a command line option (e.g. --parallel, -p) that makes (top-level) tests run in parallel. Possibly a second option (e.g. --jobs, -j) will be added to limit the number of tests running at once.

How should output be handled? Probably buffer each test's output and print it after the test has finished.

How should bailing out work? Good question! 😉 This will require some more thinking.