Enrich and sync dotnet test (VSTest) and selective unit tests docs

[azat-msft]

This PR consolidates two efforts (it supersedes and replaces #2) that enrich and cross-reference the VSTest test docs. The two contributions complement each other and touch mostly distinct sections.

docs/core/testing/selective-unit-tests.md

• Add the filter character escaping table and note that filter values can be escaped programmatically with the Microsoft.VisualStudio.TestPlatform.ObjectModel.Utilities.FilterHelper.Escape API.
• Add Category and Property to the NUnit filter properties, keeping this table consistent with the one in dotnet-test-vstest.md.

docs/core/tools/dotnet-test-vstest.md

Enrichment (originally this PR):

• Link the test platform environment variables reference from the crash-dump section.
• Add a .runsettings blame data collector example and call out CollectDumpOnTestSessionHang and MonitorPostmortemDebugger.
• Expand the --collect description to mention the Event Log collector and authoring your own collector.

Cross-reference sync with microsoft/vstest (folded in from #2):

• Correct the filter property table: xUnit Category → Traits; NUnit gains TestCategory, Category, and Property; plus a note on how xUnit traits and NUnit properties/categories map to filter expressions.
• Document the --diag tracelevel sub-option and its allowed values.
• Document setting TestRunParameters from the command line (since the .NET 5 SDK) and that RunSettings arguments take precedence over a .runsettings file.
• Fix the RunSettingsArguments link from the archived Microsoft/vstest-docs repo to the current microsoft/vstest repo.
• Bump ms.date.

This is a fork PR for user review before upstreaming to dotnet/docs.

Note

This PR was created with the assistance of AI (ai-usage: ai-assisted, already present in the articles).