Option to spew subprocess streams during tests

This supersedes PR #4268.

Run the tests with `--subproc-streams show` to show output from subprocesses (kernels, IPython.parallel components) in the terminal, or with `--subproc-streams discard` to send it to /dev/null. By default (or with `--subproc-streams capture`) the output is piped, captured and displayed only when tests fail. But in some situations, a test fails because of an error which actually occurred earlier, so you have to see all the output.