global.json template: add `--test-runner` option

[Youssef1313]

The current template for of global.json used in dotnet new globaljson is:

https://github.com/dotnet/sdk/blob/626e7bc9260c43bb68bbc9a87bfc29d163547c81/template_feed/Microsoft.DotNet.Common.ItemTemplates/content/GlobalJson/global.json

It has option to set SDK version and an option to set roll forward policy.

I think it may make sense to extend that to have an option to set test runner.

Suggested behavior:

• dotnet new globaljson -> doesn't produce the "test" section in global.json at all.
• dotnet new globaljson --test-runner <VALUE> -> produces the "test" section in global.json, and sets the runner to the value requested by the user. If <VALUE> is not VSTest or Microsoft.Testing.Platform, we consider it as an error.

[Youssef1313]

@baronfel Thoughts please?

[baronfel]

Yep, entirely reasonable