message: Fix expect((void), ...) in testing DSL
diff: @@ -267,6 +267,23 @@ private:
  };
  +template <>
  +class expect_clause<void> : public expect_clause_base<expect_clause<void>> {
  +public:
  + template <class... Us>
  + expect_clause(Us&&... xs)
  + : expect_clause_base<expect_clause<void>>(std::forward<Us>(xs)...) {
  + // nop
  + }
  +
  + void with() {
  + CAF_REQUIRE(dest_ != nullptr);
  + auto ptr = dest_->mailbox().peek();
  + CAF_CHECK(ptr->content().empty());
  + this->run_once();
  + }
  +};
  +
  template <class Config = caf::actor_system_config>
  struct test_coordinator_fixture {
  using scheduler_type = caf::scheduler::test_coordinator;
  