tests: Fix processing tests on Windows. See the comment in `pipeline.py` for more info but basically I was passing already initialized processors to the worker pool, which means pickling the whole app. Pretty bad. Interesting that it only broke on Windows, though.

import os


def slashfix(path):
    if path is None:
        return None
    if isinstance(path, str):
        return path.replace('/', os.sep)
    fixed = []
    for p in path:
        fixed.append(p.replace('/', os.sep))
    return fixed