Update test command to use global.json

[Youssef1313]

This PR removes support for dotnet.config and switches the runner/platform choice to global.json instead.

dotnet.config was a new configuration file in .NET 10 that we only used for dotnet.test.runner setting. It's available up to RC1 already (RC1 will ship with dotnet.config). This PR aims to remove dotnet.config starting with RC2. This is based on user feedback about dotnet.config

[Youssef1313]

DRAFTING to avoid accidental merge for now.

[tkapin]

Please make sure the new logic is tested. It can be done in a separate PR.

Also, please make sure that the PR is properly described.

[Youssef1313]

The change is approved.

[KalleOlaviNiemitalo]

This is based on user feedback about dotnet.config

Out of curiosity, where was this user feedback provided?

I searched for dotnet.config in issues, pull requests, and discussions in the dotnet/sdk repository, and in Visual Studio feedback, but did not find objections to it.

[Youssef1313]

@KalleOlaviNiemitalo I'm aware of some internal feedback. @Evangelink Do you know if we also got public user feedback here?

[Evangelink]

Main source was internal feedback, but we also got some from framework authors and few pings on either blogposts or videos (I can't recall).