Support custom tests

[marvinvo]

Currently, tests are bounded to the existing crysl rules. But functionality should be tested also for more general cases. This is needed, especially when thinking of other use-cases CogniCrypt could inspect.

The idea behind this new testing framework is, to define own crysl rules for own classes. Both can be designed such that writing tests for them is quite easy.

Tests for required predicate checks are included in this pr.

Before running, make sure you have the maven project with the custom classes installed. This might need a better solution in future, e.g. it should automatically install before running all tests.

[AnakinRaW]

scheduled for 2.8

[smeyer198]

I reworked some parts to include the custom classes in the test resources of the project. This has the andavantage that there is no additional project containing the custom classes anymore. This also allows us to write custom tests directly in the test directory, i.e. they are recognized by Maven without any further steps.