Add multiprocessing to quieten tests

Before we were getting
    Error in atexit._run_exitfuncs:
    Traceback (most recent call last):
      File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/atexit.py", line 24, in _run_exitfuncs
        func(*targs, **kargs)
      File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/multiprocessing/util.py", line 284, in _exit_function
        info('process shutting down')
    TypeError: 'NoneType' object is not callable
    Error in sys.exitfunc:
    Traceback (most recent call last):
      File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/atexit.py", line 24, in _run_exitfuncs
        func(*targs, **kargs)
      File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/multiprocessing/util.py", line 284, in _exit_function
        info('process shutting down')
    TypeError: 'NoneType' object is not callable

Adding multiprocessing shuts this up.
Merge pull request #8 from alphagov/fix_multiprocessing_errors

Add multiprocessing to quieten tests