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/tools/global-json.md	global.json overview
docs/core/whats-new/dotnet-10/sdk.md	1. Create a single-file C# app with a shebang

[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]

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

[IEvangelist]

Waiting for RC 2, remove the blocked label once that ships.