The spec expects `postMessage()` to act as if it is invoked
immediately. Since `postMessage()` isn't actually invoked immediately,
keep tasks with source `PostedMessage` in the task queue, so that these
tasks are processed. Fixes a hang when `WorkerGlobalScope.close()` is
called immediately after `postMessage()`.
This aligns Workers and Window and MessagePorts to all use the same
mechanism for transferring serialized messages across realms.
It also allows transferring more message ports into a worker.
Re-enable the Worker-echo test, as none of the MessagePort tests have
themselves been flaky, and those are now using the same underlying
implementation.
