Implement fixture for improved testing

[bitbeckers]

Issue
Since test cases influence contract/wallet state, we've added some quick cleanups within the test cases. This also means not all tests run independently (e.g. deposit and withdraw).

To speed up testing, make testing more flexible and be able to do quick state management, we want to implement fixtures.

For instance, since we already create a commitment in an earlier testcase, a subsequent call to create a commitment in another test case is rejected since there is already an active commitment for the user. To clear up that commitment we should be able to manually close is (not an implemented feature) or use and additional user/wallet. A fixture would quickly reset state and prevent this.

Reference
https://ethereum-waffle.readthedocs.io/en/latest/fixtures.html

Expected outcome

1. Test cases start with calling the fixture for a quick state reset