Clean up and fix tests

[lucas-manuel]

In order to prepare this project for further work I've done the following:

• Deleted spell file (no longer needed)
• Used mock Vat in unit tests (no longer need spells to rely in tests)
• Added multiple user functionality to enable more comprehensive testing
• Made small changes to Keg.sol that were discussed in RC

[NiklasKunkel]

lgtm