Restructure tests

[stuebingerb]

Currently, the test cases seem to be a bit unstructured, with classes in the form of "GitHubIssue123", packages that are not present in production code, and other classes not ending on "Test".

To get a better overview of what is actually tested, tests should be restructured as following:

• Implementation specific tests should be contained in a file with a "Test" suffix, i.e. functions in "Foo" should be tested in "FooTest" within the same package
• Specification related tests should be structured in a way that follows the specification, and ideally reference the tested part