You can override the default behavior by directly overriding the before and after args, leading to one of the
  following behaviors:

before=True: the temporary dir will always be cleared at the beginning of the test.

before=False: if the temporary dir already existed, any existing files will remain there.
after=True: the temporary dir will always be deleted at the end of the test.
after=False: the temporary dir will always be left intact at the end of the test.