Add the ability for a trait to run code before and after a test

[dennisweissmann]

Add the ability for a trait to run code before and after a test.

Motivation:

As laid out in the vision document, we want traits to become extensible, and be able to invoke code before and after a test's body is invoked.

This PR provides the foundation that will not just give test authors a lot more power and flexibility in what they can achieve with traits, but will allow us (in a future PR) to re-implement e.g. the timeout behavior to be less special-cased.

Modifications:

I added a new protocol CustomExecutionTrait which inherits from Trait, that has a single additional requirement: an execute function that provides a function that eventually executes the test body in its argument so that trait authors can invoke it in whatever way they need, ie. have the ability to execute code before and after they invoke that function.

An example of such a trait is

struct DoSomethingBeforeAndAfterTrait: CustomExecutionTrait & TestTrait {

  func execute(_ function: () async throws -> Void, for test: Test) async throws {
    something()
    try await function()
    somethingElse()
  }
}

(or see CustomTraitTest).

Result:

Test authors now have a much more flexible system to write traits, allowing them to execute custom code before and after a test body is run.

[dennisweissmann]

@swift-ci please test

[grynspan]

Most of my comments are just about documentation and naming. I think the overall shape of the PR is good.

[dennisweissmann]

@swift-ci please test

[grynspan]

Suggested change

	/// - testCase: The testCase under which `function` is being performed.
	/// - testCase: The test case under which `function` is being performed.

[FranzBusch]

This seems really cool. I am wondering if we can also provide something that can run a test in a withXXX style method. We see those more and more in structured concurrency and they don't fit into the before + after pattern.

[stmontgomery]

I am wondering if we can also provide something that can run a test in a withXXX style method. We see those more and more in structured concurrency and they don't fit into the before + after pattern.

This new API should support that use case, if I understand you correctly. Or if you don't think it will, can you elaborate?

[dennisweissmann]

@swift-ci please test

[grynspan]

I want to do an editorial pass on this type's documentation after we promote it to API, but it's fine at this point.

[dennisweissmann]

@swift-ci please test