Make sure "unit" test run fast or move to integration or stress folder

[qc00]

Since #744, most integration and stress tests will run with multiple storages, and #1014 will make them run against even more storages.

However, this was applied to a few unit tests, which makes them take much more time. The unit job now takes ~11min on Linux and ~45min on Windows, by far the slowest test category, invalidating the point of unit tests giving fast feedback.

For example, test_append_with_defragmentation is a hypothesis test exercising multiple functions with no clear emphasis on what specific behaviour it's checking, so it's not a unit test. It also takes 9-13min to run on non-in-memory storage, which makes it very expensive to run against more storage types. It should either be rewritten to test a specific thing or moved to the stress folder.