The dispatch_async approach causes Cedar to not be hooked into XCTest
in time for the xctool runner to pick it up.
Instead, we are now swizzling -[XCTestObservationCenter addTestObserver:]
and -_addLegacyTestObserver: and using them as an earlier hook to register
Cedar's test observer with the test observation center.
…xctool
Some extra context around this can be found at: https://github.com/pivotal/cedar/issues/383