[Concurrency] Add explicit test for dispatchMain() DispatchQueue.main.async + asserting

[ktoso]

Add another explicit test for using dispatchMain() and asserting the isolation.

The debug output here would be (prints not comitted);

Actor.cpp:385 expected is main
Actor.cpp:390 IS NOT on main thread
Actor.cpp:408 call checkIsolated did not crash

and it works even if the queue isn't implementing SerialExecutor because:

static void swift_task_checkIsolatedImpl(SerialExecutorRef executor) {
  // If it is the main executor, compare with the Main queue
  if (executor.isMainExecutor()) {
    dispatch_assert_queue(dispatch_get_main_queue());
    return;
  }

// before we actually go in and call `checkIsolated`

Did we miss some other edge case?

Related to rdar://110210541

[ktoso]

@swift-ci please smoke test