Leverage chispa to clean up the example test

[MrPowers]

Thanks for creating this sample project for the community!

I wrote spark-fast-tests to help with Scala testing and recently created a PySpark test helper library called chispa.

chispa provides nicely formatted error messages and actually errored out with this output until I set ignore_nullable = True:

Thanks again!

[AlexIoannides]

Sorry, but I want to keep this as high-level and dependency-free as is possible.