Add testing support for `credit` property
Add testing support for `credit` property

This is mostly implementation in the `fake_api`, the test itself is just
an `AssertEqual`.

MarauderXtreme/sipa,agdsn/sipa,MarauderXtreme/sipa,lukasjuhrich/sipa,agdsn/sipa,agdsn/sipa,lukasjuhrich/sipa,lukasjuhrich/sipa,lukasjuhrich/sipa,MarauderXtreme/sipa,agdsn/sipa