add assertion helpers for Active Job integration
add assertion helpers for Active Job integration

Using `assert` by itself without an error message leads to test output
like the following:

```
Failure:
QueuingTest#test_current_timezone_is_kept_while_running_perform_later [.../rails/activejob/test/integration/queuing_test.rb:118]:
Expected false to be truthy.
```

These added helpers provide a more specific error message so that the
reasons for test failures become more clear:

```
Failure:
QueuingTest#test_current_timezone_is_kept_while_running_perform_later [.../rails/activejob/test/integration/queuing_test.rb:118]:
Job AJ-63fe8bf6-7062-4a56-a10a-97c37fcaa6e6 was not executed
```

georgeclaghorn/rails,BlakeWilliams/rails,Edouard-chin/rails,Edouard-chin/rails,yawboakye/rails,esparta/rails,Edouard-chin/rails,MSP-Greg/rails,rails/rails,kmcphillips/rails,Edouard-chin/rails,kmcphillips/rails,esparta/rails,kddeisz/rails,kddeisz/rails,Erol/rails,rails/rails,georgeclaghorn/rails,yawboakye/rails,MSP-Greg/rails,kmcphillips/rails,Erol/rails,MSP-Greg/rails,rails/rails,BlakeWilliams/rails,esparta/rails,Erol/rails,kddeisz/rails,yahonda/rails,yahonda/rails,esparta/rails,georgeclaghorn/rails,yahonda/rails,kddeisz/rails,BlakeWilliams/rails,yahonda/rails,yawboakye/rails,Erol/rails,georgeclaghorn/rails,yawboakye/rails,BlakeWilliams/rails,MSP-Greg/rails,rails/rails,kmcphillips/rails