message: [c++] Add missing static_casts in io_manager tests
  grpc::Alarm takes a void*, but io_manager expects io_manager_tag*. Thus,
  we should have been casting alarm_completion_tag* to io_manager_tag*.
diff: @@ -61,7 +61,7 @@ class io_managerTests
  alarm_completion_tag<event> act;
  gpr_timespec deadline = gpr_time_0(GPR_CLOCK_MONOTONIC);
  - grpc::Alarm alarm(ioManager.cq(), deadline, &act);
  + grpc::Alarm alarm(ioManager.cq(), deadline, static_cast<io_manager_tag*>(&act));
  bool wasSet = act.completion_event.wait_for(std::chrono::seconds(30));
  UT_AssertIsTrue(wasSet);
  @@ -81,7 +81,7 @@ class io_managerTests
  alarms.reserve(numItems);
  for (size_t i = 0; i < numItems; ++i)
  {
  - alarms.emplace_back(ioManager.cq(), deadline, &act);
  + alarms.emplace_back(ioManager.cq(), deadline, static_cast<io_manager_tag*>(&act));
  }
  bool wasSet = act.completion_event.wait_for(std::chrono::seconds(30));
  @@ -146,7 +146,7 @@ class io_managerTests
  alarm_completion_tag<event> act;
  gpr_timespec deadline = gpr_time_0(GPR_CLOCK_MONOTONIC);
  - grpc::Alarm alarm(ioManager.cq(), deadline, &act);
  + grpc::Alarm alarm(ioManager.cq(), deadline, static_cast<io_manager_tag*>(&act));
  bool wasSet = act.completion_event.wait_for(std::chrono::milliseconds(1250));
  UT_AssertIsTrue(!wasSet);
  