message: Don't call AvailableVariables() unless we know that BeginStep succeeded
diff: @@ -85,7 +85,6 @@ TEST_F(CommonReadTest, ADIOS2CommonRead1D8)
  }
  ts = std::chrono::steady_clock::now();
  adios2::StepStatus status = engine.BeginStep();
  - auto av = io.AvailableVariables();
  Seconds timeBeginStep = std::chrono::steady_clock::now() - ts;
  begin_statuses.push_back(status);
  begin_times.push_back(timeBeginStep);
  @@ -94,6 +93,8 @@ TEST_F(CommonReadTest, ADIOS2CommonRead1D8)
  {
  break;
  }
  + auto av = io.AvailableVariables();
  +
  const size_t currentStep = engine.CurrentStep();
  EXPECT_EQ(currentStep, t);
  