Add deep-nested traversal e2e tests for dotnet test (MTP)

[Evangelink]

Summary

Follow-up to #55297 (traversal support for dotnet test with Microsoft.Testing.Platform, now merged). Adds deeper end-to-end coverage for nested traversal projects.

#55297 landed with basic and single-level-nested (diamond) traversal tests. This PR adds a three-level-deep nested traversal e2e test plus explicit coverage that non-test projects in the traversal graph are silently skipped — two cases not exercised by the merged tests.

What''s added

Test asset TraversalTestProjectsDeepNested — a three-level traversal graph:

dirs.proj               (level 0) -> Level1TestProject, level2/dirs.proj
level2/dirs.proj        (level 1) -> Level2TestProject, NonTestLibrary, level2/level3/dirs.proj
level2/level3/dirs.proj (level 2) -> DeepLeafTestProject

Test RunDeeplyNestedTraversalProject_ShouldRunEveryReferencedTestProjectAndSkipNonTestProjects (Debug + Release). Using the same deterministic per-launch marker-file technique as the existing diamond test, it asserts:

• Level1TestProject, Level2TestProject, and DeepLeafTestProject each run exactly once — proving recursion works past a single nesting level.
• NonTestLibrary (a plain class library referenced by the level-2 traversal) runs zero times and produces no error — proving non-test projects in the graph are silently skipped.
• The overall exit code is ZeroTests.

Validation

• test/dotnet.Tests builds clean against main (0 warnings / 0 errors).
• The new test passed locally on both Debug and Release (validated against the same product code now merged from Support Microsoft.Build.Traversal projects in dotnet test (MTP) #55297); the full traversal suite (basic + diamond + deep-nested) passed with no regressions.

Tests-only change; no product code touched.

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[Copilot]

Pull request overview

Adds end-to-end coverage in dotnet.Tests for dotnet test handling of deeply nested Microsoft.Build.Traversal graphs when using Microsoft.Testing.Platform test applications, including validation that non-test projects are skipped during execution.

Changes:

• Added a new deep-nested traversal test asset (TraversalTestProjectsDeepNested) with a 3-level traversal graph and a non-test class library node.
• Added a new e2e test asserting each referenced MTP test project runs exactly once across the deep traversal graph.
• Added explicit verification that a non-test project in the traversal graph produces zero launches (marker files) and does not affect the successful ZeroTests exit code.

Show a summary per file

File	Description
test/TestAssets/TestProjects/TraversalTestProjectsDeepNested/dirs.proj	Top-level traversal project referencing level-1 test project and nested traversal.
test/TestAssets/TestProjects/TraversalTestProjectsDeepNested/level2/dirs.proj	Level-1 traversal referencing a test project, a non-test library, and a deeper traversal.
test/TestAssets/TestProjects/TraversalTestProjectsDeepNested/level2/level3/dirs.proj	Deepest traversal referencing the leaf test project.
test/TestAssets/TestProjects/TraversalTestProjectsDeepNested/global.json	Configures the test asset to use Microsoft.Testing.Platform runner.
test/TestAssets/TestProjects/TraversalTestProjectsDeepNested/Level1TestProject/Level1TestProject.csproj	MTP test application project used as a traversal node.
test/TestAssets/TestProjects/TraversalTestProjectsDeepNested/Level1TestProject/Program.cs	MTP dummy adapter + marker-file emission for deterministic run counting.
test/TestAssets/TestProjects/TraversalTestProjectsDeepNested/Level2TestProject/Level2TestProject.csproj	MTP test application project used as a traversal node.
test/TestAssets/TestProjects/TraversalTestProjectsDeepNested/Level2TestProject/Program.cs	MTP dummy adapter + marker-file emission for deterministic run counting.
test/TestAssets/TestProjects/TraversalTestProjectsDeepNested/DeepLeafTestProject/DeepLeafTestProject.csproj	MTP leaf test application project reachable only via deep nesting.
test/TestAssets/TestProjects/TraversalTestProjectsDeepNested/DeepLeafTestProject/Program.cs	MTP dummy adapter + marker-file emission for deterministic run counting.
test/TestAssets/TestProjects/TraversalTestProjectsDeepNested/NonTestLibrary/NonTestLibrary.csproj	Plain class library included in traversal graph to validate silent skip (no execution).
test/TestAssets/TestProjects/TraversalTestProjectsDeepNested/NonTestLibrary/Class1.cs	Minimal code file documenting intent of non-test project node.
test/dotnet.Tests/CommandTests/Test/GivenDotnetTestBuildsAndRunsTests.cs	Adds the deep-nested traversal e2e test asserting run counts and ZeroTests exit code.

Copilot's findings

• Files reviewed: 13/13 changed files
• Comments generated: 0