FixturesManager should extend a list of filenames rather than append

[miloconway]

The doc for FixturesManager states that it accepts "list_or_str" of filenames. However, it always calls extend on the given parameter. If the input is a list, appending a list to a list gives you a list of lists. In this case self.filenames should be extending the given input.

[charlax]

Indeed. Will have a look.