Replace dotnet.config with global.json in dotnet test

[mariam-abdulla]

This pull request updates the documentation for the dotnet test command to clarify how to enable the Microsoft Testing Platform (MTP) test runner. The instructions now reference global.json instead of the deprecated dotnet.config, and the example configurations have been updated accordingly.

Documentation updates for test runner configuration:

• Updated the description to specify that enabling MTP is now done by setting the test runner in the global.json file, rather than using a dotnet.config file.
• Replaced all example configurations from INI-style dotnet.config to JSON-style global.json format.
• Updated warning messages to reference opting in via global.json instead of dotnet.config.

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Internal previews

📄 File	🔗 Preview link
docs/core/testing/unit-testing-with-dotnet-test.md	Testing with 'dotnet test'
docs/core/tools/dotnet-test.md	dotnet test command
docs/core/whats-new/dotnet-10/sdk.md	What's new in the SDK and tooling for .NET 10

[Youssef1313]

LGTM, but blocking merge until dotnet/sdk#50673 is merged.

[Youssef1313]

@mariam-abdulla Please make sure to update the other pages that mention dotnet.config.

• docs/core/whats-new/dotnet-10/sdk.md
• docs/core/testing/unit-testing-with-dotnet-test.md

[mariam-abdulla]

@mariam-abdulla Please make sure to update the other pages that mention dotnet.config.

• docs/core/whats-new/dotnet-10/sdk.md
• docs/core/testing/unit-testing-with-dotnet-test.md

PR updated

[Youssef1313]

The other PR is merged.

One small question. Should we actually briefly mention that during early previews and RC1 this was using dotnet.config, which we dropped in favor of global.json in RC2?

I want to avoid confusion when people are trying to use RC1 (because RC2 isn't released yet).

@gewarren Is there generally some policy docs follow when there is a breaking change between RC1 and RC2 when RC2 isn't yet released?

[Youssef1313]

LGTM but needs an answer on the open question, as this is a breaking change between RC1 (released already) and RC2 (not yet released).