message: Fix deadlock in producer_adapter test
diff: @@ -108,7 +108,9 @@ public:
  FAIL("unable to parse input: " << err);
  ++received_messages;
  if (auto capacity_left = adapter_->push(val); capacity_left == 0)
  + {
  down->suspend_reading();
  + }
  return static_cast<ptrdiff_t>(buf.size());
  }
  @@ -119,7 +121,6 @@ public:
  struct fixture : test_coordinator_fixture<>, host_fixture {
  fixture() : mm(sys) {
  - mm.mpx().set_thread_id();
  if (auto err = mm.mpx().init())
  CAF_FAIL("mpx.init() failed: " << err);
  }
  