Fixed GitHub Actions integration tests

[ltranu20]

Fixed integration test runner being stuck and restored the removed MySQL service.

NB! Integration tests are still broken, this only fixes tests not being ran at all on GitHub.

Description

Added xunit.runner.json with unlimited parallel threads to prevent tests getting stuck in a deadlock on a GitHub runner. Restored MySQL service as the integration tests are doing funny stuff to the database.

Fixes #242, https://github.com/aau-giraf/.github/issues/14, https://github.com/aau-giraf/.github/issues/15

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Checked the Github Actions page.

Development Configuration

• Flutter version: numbers go
• Dart version: brrrr

Checklist:

• [x] My code follows the style guidelines of this project
• [ ] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas, if necessary
• [ ] I have made corresponding changes to the documentation, if necessary
• [x] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works, if necessary
• [x] New and existing unit tests pass locally with my changes
• [ ] Any dependent changes have been merged and published in downstream modules
• [ ] I have Acceptance Tested this on an iOS device
• [ ] I have Acceptance Tested this on an Android device