feat(tests): add tests for utils.fs and utils.current_file_path()

[troiganto]

Summary

These are some tests I wrote while wrapping my head around how the path-handling utils work. 🙂

Related Issues

Extracted from #878

Changes

I've split things into multiple commits to make the review a bit easier.

• changed one function in mappings.lua from formatting headline tags manually to using utils.tags_to_string()
• changed one occurrence of vim.cmd(("edit %s"):format(file)) to vim.cmd.edit(file)
• refactored tests/plenary/helpers.lua to declare its exported functions the same way all other modules do
• added functions with_var() and with_cwd() to helpers.lua to temporarily change global state
• added tests for utils.current_file_path() function
• added tests for utils.fs module

Checklist

I confirm that I have:

• Followed the Conventional Commits specification (e.g., feat: add new feature, fix: correct bug, docs: update documentation).
• My PR title also follows the conventional commits specification.
• Updated relevant documentation, if necessary.
• Thoroughly tested my changes.
• Added tests (if applicable) and verified existing tests pass with make test.
• Checked for breaking changes and documented them, if any.

[kristijanhusak]

Thanks!