usersPath bug, unit tests (WIP)

:no_entry: :construction: work in progress

Two issues with `SharedCache::getFolderContents()`:
- the `usersPath` attribute (which is currently only used once by encryption) returns the wrong users path when sharing a sub-sub-subdir.
- the `path` attribute value is not consistent: when calling `getFolderContents('')` the path is relative t the shared storage but when called on a subdir it is an absolute path relative to the owner's subdir

This PR adds a few unit tests to demonstrate this.

@schiesbn as discussed