TST: use temporary cache for audbcards.Datacard

[hagenw]

As discovered in #97 (comment), the temporary cache for audbcards was not used when testing audbcards.Datacard. This is fixed by this pull request.

[ChristianGeng]

While this gets the test green for me,
the solution is to some extent scary as for me it is not plausible why it works:
the scope of the cache fixture is function.
This to me means that the three test cases can be perceived in isolation.

I will approve this anyway.

[hagenw]

the scope of the cache fixture is function.

Yes, which means we start with an empty cache for each test function, which means in this case with an empty cache for each call to audbcards.Datacard. For the results of the tests we have at the moment, it will not make a difference if the cache is per function or per session. The only important part is that the cache is separated from a potential user cache.