UnitTest additions

[joe-hauns]

Miscellanious refactors to unit testing, allowing for different clause pattern to check against when running tests.
i.e. the new GenerationTester can not only check whether the result of a rule application is exactly some list of clauses (ExactlyStackMatcher) but also whether e.g. the resulting list contains some element ContainsStackMatcher, and simliar.

Further now in generation testing we distinguish between symetric and asymmetric test cases. in an asymmetric test case (which was the usual old TestCase before refactoring) we fill an index with some context literals (AsymmetricTest::context()) and then check the result of calling Rule::generateClauses(input) for some input clause (AsymmetricTest::input).
In a symmetric test there is no distinction between context and input. We run a couple of asymmetric tests using every clause in SymmetricTest::input as AsymmetricTest::input and the remaining clauses as AsymmetricTest::context.