Unified testing utils, part 1

[sfc-gh-turbaszek]

Pre-review checklist

• [x] I've confirmed that instructions included in README.md are still correct after my changes in the codebase.
• [x] I've added or updated automated unit tests to verify correctness of my new code.
• [x] I've added or updated integration tests to verify correctness of my new code.
• [ ] I've confirmed that my changes are working by executing CLI's commands manually on MacOS.
• [ ] I've confirmed that my changes are working by executing CLI's commands manually on Windows.
• [x] I've confirmed that my changes are up-to-date with the target branch.
• [ ] I've described my changes in the release notes.
• [x] I've described my changes in the section below.

Changes description

First PR in series of many to standarise / unify testing utils across unit and integration test. Changes:

• use single implementation of project_directory fixture
• unify temporary and path switching utils
• removed few unused methods