Improve core testing infrastructure

[marzipankaiser]

Changes:

• Renamer also renames type parameters
• There are tests for Renamer
• Add mock Context for CoreTests
• Add CorePhaseTests which allows to write tests for phases on core with minimal boilerplate

[marzipankaiser]

With the last commit, we can now write tests for a phase on CoreTransformed by simply:

class MyPhaseTests extends CorePhaseTests(MyPhase) {
    test("my test"){
        // definitions etc
        assertTransformsTo(inputAsString, outputAsString)
    }
}