Tests: Fix database cleaner separation

[Splines]

In #647, we noticed that our unit tests were failing in the pipeline (GitHub Actions). We noticed locally that the database is not fully cleaned and there are users left between tests. We now use the slower deletion strategy which will entirely delete the tables since transactions was not working for us. This might be due to after_commitor after_save callbacks as discussed here.

Note that for the deletion strategy, we also tried out to set pre_count and reset_ids to true as described here, but this made the tests slower, so we will omit those options.

For a discussion on delete vs. truncate, see this StackOverflow answer.

Note that this PR was opened after a long night session were Denis and I both tried to dig down deep. We hope this PR doesn't break anything 😅 But it shouldn't and if it does, then only our tests.