Fix caching to be testable.
    
    The old way didn't work because our ``cache`` object was global.  It
    would get configured by the first test and then never be reconfigurable
    for other tests.
    
    This gets fixed here by hanging the cache region on `flask.current_app`
    so that it gets reinstantiated and reconfigured for each test run (and
    each WSGI process).