Support artifacts output mode for MTP test results

[Evangelink]

Summary

• read UseArtifactsOutput, ArtifactsPath, ArtifactsProjectName, and ArtifactsPivots from evaluated MTP test projects
• default test artifacts to <ArtifactsPath>/test/<project>/<pivot> when artifacts output is enabled
• preserve explicit --results-directory and --results-directory-layout choices
• place post-processed reports in the shared artifacts test directory

The test category is a sibling of bin, obj, package, and publish; it can contain reports, coverage, diagnostics, and other generated test artifacts without implying every file is a test result.

Fixes #54612

Validation

• full build.cmd
• 52 focused dotnet.Tests covering artifacts defaults, explicit layout precedence, collisions, and artifact post-processing

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[Copilot]

Pull request overview

This PR updates dotnet test’s Microsoft Testing Platform (MTP) integration to recognize MSBuild’s artifacts output mode and default test results into the artifacts layout when enabled, while preserving explicitly specified --results-directory / --results-directory-layout behavior. It also ensures artifact post-processing outputs land in the shared artifacts results directory.

Changes:

• Plumb UseArtifactsOutput, ArtifactsPath, ArtifactsProjectName, and ArtifactsPivots from evaluated test projects into the MTP TestModule model.
• Update results-directory resolution to default to <ArtifactsPath>/test-results/<project>/<pivot> when artifacts output is enabled (unless the user explicitly overrides results directory/layout).
• Adjust artifact post-processing output directory selection and expand test coverage for artifacts output defaults and precedence rules.

Show a summary per file

File	Description
test/dotnet.Tests/CommandTests/Test/TestResultsDirectoryResolverTests.cs	Adds unit tests covering artifacts output defaults and explicit layout precedence.
test/dotnet.Tests/CommandTests/Test/TestCommandParserTests.cs	Extends parser tests to validate the new ResultsDirectoryLayoutSpecified signal.
test/dotnet.Tests/CommandTests/Test/GivenDotnetTestBuildsAndRunsTests.cs	Adds an integration test verifying artifacts output places reports under artifacts/test-results and avoids TestResults.
test/dotnet.Tests/CommandTests/Test/ArtifactPostProcessingManagerTests.cs	Adds coverage for artifact-mode output directory selection; refactors job construction for module injection.
src/Cli/dotnet/Commands/Test/MTP/TestResultsDirectoryResolver.cs	Implements artifacts-aware default root/layout behavior and introduces ResultsDirectoryLayoutSpecified-gated defaults.
src/Cli/dotnet/Commands/Test/MTP/SolutionAndProjectUtility.cs	Reads artifacts-related MSBuild properties and stores them into TestModule.
src/Cli/dotnet/Commands/Test/MTP/Options.cs	Adds ResultsDirectoryLayoutSpecified to PathOptions.
src/Cli/dotnet/Commands/Test/MTP/MSBuildUtility.cs	Sets ResultsDirectoryLayoutSpecified based on whether --results-directory-layout was explicitly provided.
src/Cli/dotnet/Commands/Test/MTP/Models.cs	Extends TestModule to carry artifacts output metadata.
src/Cli/dotnet/Commands/Test/MTP/ArtifactPostProcessingManager.cs	Prefers the artifacts results directory as the merge output location when artifacts output is enabled.
src/Cli/dotnet/Commands/Test/CliConstants.cs	Adds MSBuild property name constants for artifacts output mode integration.

Copilot's findings

Suppressed comments (1)

src/Cli/dotnet/Commands/Test/MTP/TestResultsDirectoryResolver.cs:236

• This comment claims the pivot deliberately excludes configuration, but when artifacts output defaults are used the pivot comes from $(ArtifactsPivots), whose default value includes configuration (for example: debug_net8.0). This mismatch makes the behavior harder to reason about and can confuse future changes.

    /// The pivot folder distinguishing runs of the same project across target frameworks and
    /// runtimes. Multiple elements are joined by an underscore, following the artifacts layout.
    /// The configuration is deliberately not part of the pivot: a single test run targets one
    /// configuration, so it would only ever add a constant level to every path.
    /// </summary>

• Files reviewed: 11/11 changed files
• Comments generated: 1

[Evangelink]

@baronfel FYI