Handling files in tests #264

As a Code PushUp contributor, I want to be able to add new stable tests. Currently, the tests are flaky and handling files requires multiple mocks and setups.

Additionally, a cwd mock setup should be included in the memory fs setup to point at its root.
Remove MEMFS_VOLUME variable declared inside a test file, either hardcode or import directly from testing-utils.

This issue is a part of #145

E2E tests

• Should mostly use actual files.
• We should agree on the strategy - either timestamps or running the tests sequentially.

Integration tests

• Should almost always use memory file system, unless the core functionality requires actual files.
• The creation and removal of files created by tests should be done in a way that tests can run in parallel and no previous state is required.
• Due to flakiness, a whole folder or .gitkeep should not be a part of the repository and within tests, only the folder content is removed, not the folder itself.

Unit tests

• Should always use memory file system.
• There should be one shared setup which will be used for all unit tests using the memory file system.