Defer simulation of callback blocks until the next main thread spin, so that apps that set blocks immediately after beginFetch see the same behavior in test as in the real fetches.
Defer simulation of callback blocks until the next main thread spin, so that apps that set blocks immediately after beginFetch see the same behavior in test as in the real fetches.

google/gtm-session-fetcher,google/gtm-session-fetcher,thomasvl/gtm-session-fetcher,thomasvl/gtm-session-fetcher,thomasvl/gtm-session-fetcher,google/gtm-session-fetcher