DRAFT: xUnit Parallelize tests per scenario

[Tiberriver256]

Hello 👋

This PR is mainly to start a discussion and provide a small PoC on how this idea might be implemented.

The classes are taken and modified slightly from this excellent xUnit plugin.

Some of the questions/thoughts I had

• This PR has everything parallel by scenario unless a collection is specified using a collection attribute. Should we have this opt-in instead so it's non-breaking? I want to enable it globally somehow if it's opt-in.
• I flagged this as breaking because people may have their dependency injection set up to share data between test scenarios, or they may be expecting their scenarios to be running in sequential order within a feature; this would change that.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue).
• [ ] New feature (non-breaking change which adds functionality).
• [x] Breaking change (fix or feature that would cause existing functionality to not work as expected).
• [x] Performance improvement
• [ ] Refactoring (so no functional change)
• [ ] Other (docs, build config, etc)

Checklist:

• [ ] I've added tests for my code. (most of the time mandatory)
• [ ] I have added an entry to the changelog. (mandatory)
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.