Avoid reassigning task-local runtime state in a Runner's event handler unless it's nested

[stmontgomery]

This is a mitigation for a bug in which an expectation inside a withTaskGroup closure can cause a crash if it fails.

Motivation:

It can sometimes be useful to perform expectations (e.g. #expect) within the body of the closure passed to withTaskGroup or similar APIs. However, doing so currently causes a crash if the expectation fails, illustrated by this small example:

@Test func example() async {
  await withTaskGroup(of: Void.self) { _ in
    #expect("abc" == "123") // 💥
  }
}

Modifications:

This change avoids wrapping the event handler of a runner's configuration with one that reassigns a TaskLocal (potentially triggering the crash) unless that runner is running during the run operation of another runner. In other words, only mutate the TaskLocal for nested runners. Nesting, or running one runner in the context of another, is something which is only done in practice when performing tests of the testing library itself. Ordinary usage of the testing library only has one “outer“ runner.

Result:

The above example code no longer crashes, verified by the new unit test.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

Resolves rdar://126132392

[stmontgomery]

@swift-ci please test

[stmontgomery]

@swift-ci please test

[ktoso]

LGTM, thank you!