ci: Create CI/CD pipeline for testing packaging process; include integration tests

[SoloSynth1]

This PR:

• adds pytest plugins pytest-cov pytest-dotenv as dev dependencies
• adds integration tests which are equivalent to running fixml evaluate and fixml generate against 3 github repos (lightfm, group8 and group21)
• creates a new GitHub Action ci-full.yml to run the integration tests when pushing to main branch.
• adds extra steps in current CI pipeline to build a wheel file, then to check if it can be installed successfully in a brand new environment.

Now when running pytest, all tests including integration tests would be executed. If you want only to perform unit tests, run pytest -m "not integration". This instruction has been included in README.md.

Please note that this PR is based on changes on #164 as there were some changes in the test folder structure. You might want to review and merge that PR first.

Closes #166.

[JohnShiuMK]

looks good to me, thanks!