dqn test miss variable and is broken

The unit tests defined for the dqn example is missing `MEMORY_BATCH_SIZE`. However, even after I defined it to say 1 or 2 it still fails.

https://github.com/GoogleCloudPlatform/training-data-analyst/blob/master/quests/rl/dqn/trainer/model_test.py

it fails at
```
self.agent.learn()
```
with:
```
ValueError: Error when checking input: expected frames to have shape (4,) but got array with shape (2,)
```

Thanks a lot by the way for these great modules and for great [intro to RL course in trading specialization on Coursera!](https://www.coursera.org/learn/trading-strategies-reinforcement-learning/home/welcome) This issue may be more relevant to @danideter