Fix: Update dependency versions and add spark-testing-base to share the same spark context accross the tests

[n-raymond]

Hello !

I recently started to learn Spark and found your amazing exercises base. Since the dependency were not updated for a long time, I tried to do it myself and finally get the test working with the latest spark version.

I would like to share this modification with you if you are interested.

Fixes:

• Update dependencies versions to get an up-to-date project
• Add spark-testing-base to share the same spark context across the tests (since a unique spark context can only be instantiated at runtime) and refactor the test and classes to be compatible with this implementation.
• Replace also each occurence of null by ??? to get a NotImplementedException which is more accurate.

Thank you for your tutorials and have a really nice day !

Nicolas