Allow a trailing CancellationToken parameter on MSTest test methods #11986

Summary

Allow MSTest test methods to declare one additional trailing CancellationToken parameter. MSTest supplies TestContext.CancellationToken automatically, so a non-parameterized test can take one parameter and a parameterized test with X data arguments can take X + 1 parameters.

Background and Motivation

Tests currently need access to TestContext just to forward its cancellation token to asynchronous operations. A method parameter makes cooperative cancellation explicit and avoids that boilerplate.

Proposed Feature

[TestMethod]
public async Task TestAsync(CancellationToken cancellationToken)
{
    await RunAsync(cancellationToken);
}

[TestMethod]
[DataRow(42)]
public async Task ParameterizedTestAsync(int value, CancellationToken cancellationToken)
{
    await RunAsync(value, cancellationToken);
}

• Inject the exact current TestContext.CancellationToken, preserving its existing cancellation and timeout lifetime.
• Support ordinary methods and data-driven methods, including DataRow and DynamicData.
• Keep discovery, reflection/source-generated execution, argument validation, and MSTest analyzers consistent.
• Preserve existing test signatures and data binding; do not inject arbitrary parameters or change lifecycle method signatures.
• Add regression coverage for token identity, cancellation, data arguments, invalid signatures, and existing behavior.

Alternative Designs

Continue using TestContext.CancellationToken inside the test body. No new attribute or general dependency-injection mechanism is needed for the proposed convenience.