Forbid writes to filesystem from unit tests

[arrayka]

1. Remove all usages of std::fs::* from unit tests
2. Don't use FileStorageProvider in unit tests.
3. Use either VirtualStorageProvider::new_memory() when no access to existing files is needed, or VirtualStorageProvider::new_overlay() when read access to existing files is needed.

Two optional steps - investigate if it is feasible, if so, then implement:
4. Forbid usage of std::fs::* in Clippy.toml, use FileStorageProvider instead.
5. Put FileStorageProvider behind cfg(not(any(testing,virtual_storage)))