Fix multiprocessing tests on macOS.

[tjni]

On macOS, the default method for creating a new process is to use spawn instead of fork. When this occurs, arguments to the process target need to be pickled, but the test includes several unpicklable objects, such as local functions and mmaps.

We can fix the test's current assumptions by forcing "fork" instead of "spawn". I'm also open to fixing the test in other ways if we want to test that spawn works as well.

For reference, the original error signature:

obj = <Process name='Process-129' parent=38670 initial>, file = <_io.BytesIO object at 0x102294450>, protocol = None

    def dump(obj, file, protocol=None):
        '''Replacement for pickle.dump() using ForkingPickler.'''
>       ForkingPickler(file, protocol).dump(obj)
E       AttributeError: Can't pickle local object 'BaseTestReader._check_concurrency.<locals>.lookup'

[oschwald]

Thanks! This seems like a good immediate fix and seems to match the original intent of the test.