Add RuntimeTestMode for the coreclr interpreter

[kg]

Needed to disable tests in dotnet/runtime

To double check:

• The right tests are in and the right validation has happened. Guidance: https://github.com/dotnet/arcade/blob/main/Documentation/Validation.md

[kg]

Do I need to manually trigger this to flow? It looks like based on the arcade docs it might be automated, but I can't tell for sure.

[akoeplinger]

no it will automatically flow, first from arcade -> VMR and then VMR -> runtime. Watch for PRs like dotnet/runtime#119873 to see when the change lands there.