Infra/380.integration tests.xunit

[ForNeVeR]

• Closes Convert the integration tests to unit ones #380.
• Closes Add Test Adapter for Cesium Integration tests #428 (supersedes).

TODO

• Investigate local dead(?)locks with Medallion Shell when running dotnet test
  • Command.Result blocks indefinitely in xunit madelson/MedallionShell#104
• Rewrite the fixture to use IAsyncLifetime
• Make sure it runs on CI
• Clean up the debug code in .yml
• Remove old run modes from CI
• Make the tests green
• Finish .NET Framework support
• Experiment with parallel testing more (figure out if there's a way to set up the max parallelism at least for the command-line run)
  • Report to https://youtrack.jetbrains.com/issue/RSRP-484400
• TODO sweep

[ForNeVeR]

Thanks to @kant2002 for the implementation of the VS environment discovery and for motivation to work on this issue :)