Allow parallel runner scheduling

[MattWindsor91]

I had an idea last night: if we ever have recipes that, when run, don't take up all of the cores on a machine, we can bin-pack the recipes such that we run as many of them as possible while making each of them serve as stress for the other.

We currently have the ability to run parallel tests on the runner, but it literally just worker-pools recipes and that usually results in thrashing that slows all of the tests down.