Add nunit tests and benches

[KodrAus]

So xunit isn't working with F# on core yet, so I should use nunit instead so some tests can be written. Benchmarks would also be helpful, but how they'll play with timeouts etc is going to be a bit trickier... It seems logical that a Deadline(0) should run timeout on the next loop iteration.

[KodrAus]

NUnit isn't working either, I've just dropped back to a really simple method of writing each integration test as a process and running them with a shell script...

This isn't going to help track down memory issues though since the resources are going to be freed when the process ends regardless of whether the app freed them properly itself or not.