untangle & simplify tests

the archiver tests (also fuse tests, iirc) are way too complex, testing too much stuff in a single test.

so, let's add way more simple tests, only testing one specific and small thing per test, using pytest and fixtures.

tests requiring (fake)root should be separate and only contain the stuff requiring root.

it's a lot of work, so do it step by step:
- focus on one topic, write new tests
- do platform testing with them
- repeat

Start by reading the archiver tests and the fuse tests, then identify one focused topic to separate using pytest and fixtures. Keep tests requiring fake root isolated, run the platform tests, and consider the topic complete when each small behavior is covered independently and the platform checks pass.

Written by the indexing model from the issue text.