Adding extensive tests for releases

[kurpicz]

While we want to reduce the running time of our tests during development as much as possible, see #189, we also want thorough testing for releases. To this end, we have to add some additional extensive tests that are only run on demand, e.g. using some CMake flag.

• [ ] Make some tests optional during compile time,
• [ ] Put existing big tests in the optional files (e.g., some of the tests for #128), and
• [ ] Add bigger tests for all things that are only lightly tested right now (most of the tests are lightweight).