Enable safe parallelization for task test projects

[Evangelink]

These task-focused MSTest projects currently run serially even though most test classes do not share mutable state. Enabling conservative class-level parallelism should reduce execution time while preserving isolation for the small set of process-global resources.

Changes

• Enable ClassLevel parallelization in four task test projects.
• Serialize all classes that change the process current directory with WellKnownResources.CurrentDirectory.
• Serialize the publish environment-variable test and restore its original value from a finally block.
• Keep tests within each class sequential, avoiding races in class-private fixtures and static test data.

No DoNotParallelize exclusions are needed because the resource locks cover the complete affected class lifecycle without blocking unrelated classes.

Testing

Local test and benchmark runs were intentionally deferred so the parallelization changes can be measured together in one consolidated 10-run baseline and changed fleet benchmark.

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[Copilot]

Copilot review overview

Review tier: Lite
Findings: None

What changed in this PR

This PR enables conservative MSTest class-level parallel execution for several task-focused test projects, and adds targeted serialization via [ResourceLock] for tests that mutate process-global state (current directory / environment variables) to keep parallel runs isolated.

Changes:

• Enable MSTestParallelizeScope=ClassLevel in four MSTest test projects.
• Add [ResourceLock(WellKnownResources.CurrentDirectory)] to test classes that call Directory.SetCurrentDirectory(...).
• Serialize the publish environment-variable test with [ResourceLock(WellKnownResources.EnvironmentVariables)] and restore the original value via try/finally.

File	Description
test/​sdk-tasks.Tests/​sdk-tasks.Tests.csproj	Opt into MSTest class-level parallelization.
test/​Microsoft.NET.Sdk.Publish.Tasks.Tests/​Microsoft.NET.Sdk.Publish.Tasks.Tests.csproj	Opt into MSTest class-level parallelization.
test/​Microsoft.NET.Sdk.Publish.Tasks.Tests/​EnvironmentHelperTests.cs	Serialize env-var mutation and restore original value reliably.
test/​Microsoft.NET.Build.Tasks.Tests/​Microsoft.NET.Build.Tasks.Tests.csproj	Switch from assembly-serial execution to class-level parallelization.
test/​Microsoft.NET.Build.Tasks.Tests/​GivenTasksUseAbsolutePaths.cs	Serialize CWD-mutating test class via WellKnownResources.CurrentDirectory.
test/​Microsoft.NET.Build.Tasks.Tests/​GivenAWriteAppConfigWithSupportedRuntimeMultiThreading.cs	Serialize CWD-mutating test class via resource lock.
test/​Microsoft.NET.Build.Tasks.Tests/​GivenATaskEnvironmentDefault.cs	Serialize CWD-mutating test class via resource lock.
test/​Microsoft.NET.Build.Tasks.Tests/​GivenAResolveRuntimePackAssetsTask.cs	Serialize CWD-mutating test class via resource lock.
test/​Microsoft.NET.Build.Tasks.Tests/​GivenAResolvePackageFileConflictsMultiThreading.cs	Serialize CWD-mutating test class via resource lock.
test/​Microsoft.NET.Build.Tasks.Tests/​GivenAResolvePackageAssetsMultiThreading.cs	Serialize CWD-mutating test class via resource lock.
test/​Microsoft.NET.Build.Tasks.Tests/​GivenAResolveAppHostsMultiThreading.cs	Serialize CWD-mutating test class via resource lock.
test/​Microsoft.NET.Build.Tasks.Tests/​GivenAPickBestRidMultiThreading.cs	Serialize CWD-mutating test class via resource lock.
test/​Microsoft.NET.Build.Tasks.Tests/​GivenAGetPackagesToPrune.cs	Serialize CWD-mutating test class via resource lock.
test/​Microsoft.NET.Build.Tasks.Tests/​GivenAGenerateRuntimeConfigMultiThreading.cs	Serialize CWD-mutating test class via resource lock.
test/​Microsoft.NET.Build.Tasks.Tests/​GivenAGenerateDepsFileMultiThreading.cs	Serialize CWD-mutating test class via resource lock.
test/​Microsoft.NET.Build.Tasks.Tests/​GivenAGenerateClsidMapMultiThreading.cs	Serialize CWD-mutating test class via resource lock.
test/​Microsoft.NET.Build.Tasks.Tests/​GivenAGenerateBundleMultiThreading.cs	Serialize CWD-mutating test class via resource lock.
test/​Microsoft.NET.Build.Tasks.Tests/​GivenACheckForUnsupportedWinMDReferencesMultiThreading.cs	Serialize CWD-mutating test class via resource lock.
test/​Microsoft.NET.Build.Extensions.Tasks.Tests/​Microsoft.NET.Build.Extensions.Tasks.Tests.csproj	Opt into MSTest class-level parallelization.
test/​Microsoft.NET.Build.Extensions.Tasks.Tests/​GivenAGetDependsOnNETStandardMultiThreading.cs	Serialize CWD-mutating test class via resource lock.