Split dotnet test docs

[Evangelink]

Summary

Split dotnet test docs into sub pages rather than tabs to simplify understanding and browsability.

Fixes #50234

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Internal previews

📄 File	🔗 Preview link
docs/core/tools/dotnet-test-mtp.md	dotnet test with Microsoft Testing Platform
docs/core/tools/dotnet-test-vstest.md	dotnet test command with VSTest
docs/navigate/tools-diagnostics/toc.yml	docs/navigate/tools-diagnostics/toc