Move code only used for testing into test-fixture source

[jessicaKoehnke]

We decided that we want to use a new source test-fixtures for all code that is only needed for testing but is not a test itself. That way we don't mix production code with test code and it is easier to not accidentally use methods in production that were only ever intended for testing purposes. The test-fixtures source was added in PR #6904.

Now we need to migrate all existing test support code into the new test-fixtures source. Some basic rules:

• Create a ObjectTestFactory class for code that generates test data. For example VertexLinkerTestFactory
• The TestFactory classes should live in the same package as the objects they create. That way package private constructors can be used and it is easier to find them again in the future
• Test Implementations of interfaces should live in the same package as the interface