Add parallelism to test_pr

[bfroehle]

I often have several unused cores sitting around, so it'd be great if the test_pr script could allow some parallelism.

1. If the virtual environments were created per PR request, I'd be able to run test_pr against multiple PRs simultaneously.
2. With a slight refactoring (potentially adding locks around some bits of the code), it should also be possible to run the iptest scripts in each virtual environment concurrently.

[takluyver]

@minrk : I've always avoided running more than one instance of the test suite at a time, because I'm not sure how it would affect tests that require interprocess communications, especially in IPython.parallel. Is it actually quite safe to do that, or would it be simple to make it safe?

[minrk]

No, it is not safe to do that with the parallel stuff. I think it should be if the IPYTHONDIR is guaranteed to be different for each env.

[fperez]

What I'd like to do is rig iptest so that each 'test group' is run not only in a new process (which is already the case) but with a new, random and temporary IPYTHONDIR. That is really easy to do, and it will allow us to parallelize not only across test_pr environments but also across test groups in a single iptest run (with some attention paid to the collection of stdout/stderr, obviously, as is always the case with parallelizing console jobs).

[minrk]

That would be excellent. @bfroehle any interest in doing that here?