Allow inheriting test functions from superclasses used as test suites

[grynspan]

This PR adds experimental functionality that allows you to write a test function in a test suite of class type, and then inherit that test function in subclasses of the test suite automatically. For example:

class AnimalTests {
  func makeNoise() -> Noise? {
    return nil // nothing by default
  }

  @Test open func `Animal makes noise`() {
    #expect(self.makeNoise() != nil)
  }
}

class DogTests: AnimalTests {
  override func makeNoise() -> Noise? {
    return .bark
  }
}

class CatTests: AnimalTests {
  override func makeNoise() -> Noise? {
    return .meow
  }
}

We leverage the open keyword for this purpose for a few reasons:

1. It has no semantic meaning in test suites today, so it's unlikely to be in use already.
2. It implies that there will be subclasses of the class in which it is found.
3. The compiler enforces that it can only be applied to member functions of classes, and can do so with type system knowledge rather than just syntax.

We could spell it differently, e.g. with another attribute macro like @inherited, but we wouldn't be able to statically determine when it is incorrectly applied to something that isn't a member function of a class. So for the purposes of the experimental feature, open it is!

This makes the override keyword ambiguous, so I've blocked it when used with @Test, but there is probably a way to resolve that so we should consider that constraint to be temporary.

To try this feature out, set the Swift build condition SWIFT_TESTING_EXPERIMENTAL_TEST_INHERITANCE_ENABLED in your test target.

Checklist:

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