Allow users to choose the number of processors used for running tests.

Added an extra argument to the test runner command (`--num-processors`/`-p`) which allows the user to choose the number of processors (or rather processes I suppose) to use for testing.

This also required a minor modification to the runnable part of the `iris.tests.runner` package since it was not wired to cope with the trailing `=` that setuptools requires to recognise an argument that takes a value.