message: Fix bug in testing DSL's next_mailbox_element
  The function next_mailbox_element did not deal with blocked actors
  correctly, causing hard-to-understand errors in unit tests.
diff: @@ -205,11 +205,12 @@ private:
  inline caf::mailbox_element* next_mailbox_element(caf_handle x) {
  CAF_ASSERT(x.get() != nullptr);
  auto sptr = dynamic_cast<caf::scheduled_actor*>(x.get());
  - if (sptr != nullptr)
  - return sptr->mailbox().peek();
  + if (sptr != nullptr) {
  + return sptr->mailbox().blocked() ? nullptr : sptr->mailbox().peek();
  + }
  auto bptr = dynamic_cast<caf::blocking_actor*>(x.get());
  CAF_ASSERT(bptr != nullptr);
  - return bptr->mailbox().peek();
  + return bptr->mailbox().blocked() ? nullptr : bptr->mailbox().peek();
  }
  // -- introspection of the next mailbox element --------------------------------
  