Modified 'koch tests' to use the nimrod binary in the bin directory

I added the option to specify what nimrod binary testament is supposed to use to conduct its tests, and changed koch tests to use this option to target the nimrod in the bin path.
The way I had the tester set the binary to use (by setting an environment variable) is a bit of a hack. If it's felt that it shouldn't be done that way, let me know and I'll fix it.