[FLINK-7415][Cassandra Connector] Add comments for cassandra-connector test classes

[yew1eb]

What is the purpose of the change

Help beginners follow it with less confusion.

Brief change log

• Add comments for cassandra test classes

Verifying this change

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (no)

Documentation

• Does this pull request introduce a new feature? (no)

[yew1eb]

I ran the test class with an exception, I found the cause finally.

[zentol]

+1.

[zentol]

merging.