untangle & simplify tests

[ThomasWaldmann]

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

also, they use unittest style (see #28).

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
• rip out that one topic from existing unittest style tests
• repeat