Refactor SDF test

[MatthijsBurgh]

Simplest way to fix is to create a clean new object for each test, and then get it in the proper state for each test by calling whatever previous tests you need to call first.

Obviously, a test that is also used in another test is useless as test by itself since its test is also performed by "another test". It's a "fake test", looks good in test count, but it is not testing anything that isn't covered by other tests already.
You can simply drop those fake tests as test (you still need the code for the "another test" that uses it though). That should also be a simple thing to change.

The complicated part comes in finding better tests, which do proper testing. Making a new issue for it is likely useless, as nobody is going to fix that I think.

Originally posted by @alberth in #12 (comment)