.NET: CI Build time end to end improvement

[westey-m]

Motivation and Context

Our builds are slow and needs improvement.

Description

• Upgrade to XUnit 3 and Microsoft Testing Platform to allow more parallel execution options
• Split Build and Test into parallel Jobs, so that Test only needs to build src and tests and not samples
• Instead of running tests in a loop with filtering per project, we generate a new filtered slnx upfront, which we can point the test runner at, which runs all tests in parallel.
• Builds with integration tests are down from ~25m to ~13m.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.