Don't call a notification expectation handler after the expectation has expired

[briancroom]

Previously, if a notification expectation fails, either due to a waitForExpectationsWithTimeout call or because the test ends, the internal notification observer could still end up calling the expectation handler if the expected notification is fired at a later point. With this change, the notification observer weakly captures the expectation, and won't do any work if the notification is observed but the expectation isn't around anymore.

[mike-ferris]

@swift-ci please test

[modocache]

Good catch, thanks @briancroom!