When running global before and after hooks, we were picking up any class that implented +beforeEach and/or +afterEach. You could opt-out by conforming to the SPTExcludeGlobalBeforeAfterEach protocol. This could cause difficult to debug failures when it picked up imported classes. This commit reverses the approach, creating a SPTIncludeGlobalBeforeAfterEach protocol, which SPTExampleGroup implements. Now we only select classes that implement SPTIncludeGlobalBeforeAfterEach for running global +beforeEach and/or +afterEach. You can opt-in by implementing the protocol.