Add more equals tests for Property

In #177 I said "more to come" which I never did. Property still misses some tests, at least an equals test. What about this?
You are right, but the test below does a little bit more than the data provider could do. I think this is good enough as it is and did not wanted to waste more time. Please note that we added the `testPropertiesWithDifferentStatementsAreNotEqual` because of an actual bug we once had.