Replace most subprocess-spawning tests w/ dependency injection

[bitprophet]

Many tests that are testing specific behavior of e.g run() take the easy route and call e.g. run('true', xxx), then introspect the result. This of course runs an actual subprocess and makes for a slower test suite ("only" ~4s right now, but it will keep growing.) Some even spawn a subprocess that spawns another subprocess! (E.g. the end of #21's new tests do this to get around pty stuff.)

Only the highest level sanity/integration tests really need to do this; the rest should be able to work with a dependency-injected mock object that behaves as needed w/o spawning a real subprocess.

Required:

• Choose a mocking lib if we aren't already using one.
• Change run to accept a runner argument & factor out the pexpect/subprocess using chunk of current function body.
• Update most tests to do run(runner=<mock>).

[bitprophet]

Completely forgot about this ticket - I took care of this before tackling #120 / #123 and the test suite went from ~9s to ~3s. Most of what remains are actual tests of run itself which I am loathe to toss out into the integration suite as they're crucial. Let's call this done for now. If test time grows again we can make a 2nd pass later.