message: Cleanup expect_clause and test fixture
  Remove unused member `mock_dest_`
  Test peek_mailbox for `!= nullptr`
  Cancel all pending timeouts in fixture destructor
diff: @@ -163,7 +163,6 @@ class expect_clause_base {
  public:
  expect_clause_base(caf::scheduler::test_coordinator& sched)
  : sched_(sched),
  - mock_dest_(false),
  dest_(nullptr) {
  // nop
  }
  @@ -201,14 +200,7 @@ public:
  return dref();
  }
  - Derived& to(const wildcard& whom) {
  - CAF_REQUIRE(sched_.prioritize(whom));
  - dest_ = &sched_.next_job<caf::scheduled_actor>();
  - return dref();
  - }
  -
  Derived& to(const caf::scoped_actor& whom) {
  - mock_dest_ = true;
  dest_ = whom.ptr();
  return dref();
  }
  @@ -217,6 +209,7 @@ public:
  std::tuple<const Ts&...> peek() {
  CAF_REQUIRE(dest_ != nullptr);
  auto ptr = peek_mailbox(dest_);
  + CAF_REQUIRE(ptr != nullptr);
  if (!ptr->content().template match_elements<Ts...>()) {
  CAF_FAIL("Message does not match expected pattern: "
  << to_string(ptr->content()));
  @@ -237,10 +230,7 @@ protected:
  return *static_cast<Derived*>(this);
  }
  - // denotes whether destination is a mock actor, i.e., a scoped_actor without
  - // functionality other than checking outputs of other actors
  caf::scheduler::test_coordinator& sched_;
  - bool mock_dest_;
  caf::strong_actor_ptr src_;
  caf::local_actor* dest_;
  std::function<void ()> peek_;
  @@ -350,7 +340,6 @@ class disallow_clause_base {
  public:
  disallow_clause_base(caf::scheduler::test_coordinator& sched)
  : sched_(sched),
  - mock_dest_(false),
  dest_(nullptr) {
  // nop
  }
  @@ -386,7 +375,6 @@ public:
  }
  Derived& to(const caf::scoped_actor& whom) {
  - mock_dest_ = true;
  dest_ = whom.ptr();
  return dref();
  }
  @@ -405,10 +393,7 @@ protected:
  return *static_cast<Derived*>(this);
  }
  - // denotes whether destination is a mock actor, i.e., a scoped_actor without
  - // functionality other than checking outputs of other actors
  caf::scheduler::test_coordinator& sched_;
  - bool mock_dest_;
  caf::strong_actor_ptr src_;
  caf::local_actor* dest_;
  };
  @@ -512,6 +497,10 @@ struct test_coordinator_fixture {
  // nop
  }
  + ~test_coordinator_fixture() {
  + sched.clock().cancel_all();
  + }
  +
  template <class T = int>
  caf::expected<T> fetch_result() {
  caf::expected<T> result = caf::error{};
  