tests: Improve smart remove related unit tests

[buhtz]

Followup of PR #1819 and Issue #1297 .

Refactor and improve the existing unit tests about smart remove. Add new tests if appropriate.
Some tests do exist:

/common/test/test_snapshots.py::TestSnapshots.test_smartRemove*()

[emtiu]

It's my impression that the existing unittests actually assert the buggy behavior that will be fixed by #1819. But I will need to take a pen and paper and carefully make sure I understood it right :)