Mock file system code

[PatrickKa]

Description

I am not sure if we have time for tests that would require this but in principle, mocking the file system would be useful for the EDU, RF and command stuff.

[PatrickKa]

Blocked by #186 and #195

Edit: We can mock on the fly while implementing the file system code so this is no longer blocked by the above two issues

[PatrickKa]

We already have a littlefs implementation that works on Linux and stores everything in RAM. This should be sufficient for most (low-level) tests. However, it does not provide the full mocking capabilities described in the parent issue (#209). Furthermore, we should actually mock the high-level file system functions that are mentioned in #195. Therefore, this issues stays open for now.