test: standardize unity project setup

[ComradeVanti]

This PR standardizes Unity project setup procedures for tests. This was previously done using the functions inside mock-work-dir. The PR changes and expands these functions in a few ways.

Changes

The new system always sets up the same folder structure which looks like this:

[Os Temp folder]
| test-openupm-cli
   | .upmconfig.toml
   | Project
      | ProjectSettings
         | EditorVersion.txt
      | Packages
         | manifest.json

This is equivalent to the old test-project structure except for the addition of a .upmconfig.toml. The test-openupm-cli folder now serves as a mocked user-home, which is why the config is stored there. Accordingly, for the duration of the tests the HOME environment variable will be set to the path of test-openupm-cli. The content of the .upmconfig.toml is configurable for different tests.

Common utility functions such as, resetting the folder between tests, querying the content of the project manifest or getting the path to the project directory have also been standardized.

In order to prevent needing to override cwd for each tests, process.cwd() is mocked to be inside test-openupm-cli/Project.

Reason

This standardization will future proof for three things which may be done in the future:

• Allow easier implementation of configuration extensions for mock projects.
• Use mock-fs or a similar library in order to run tests off the real file-system. This will allow to more easily parallel tests.
• Prepare for a similar setup to be implemented for Unity package folders, in case OpenUpm also adds functionality for working with packages directly (init, publish or similar)