Write unit tests for listenbrainz stats module

 _init_.py would be a good start for writing tests.  SparkTestCase can be used as the base class.

Keep in mind the following:

• unit tests aim at testing functions independently. So write tests for all functions of the file you are testing.
• Data used in testing is mostly constant and should be destroyed as the tests end.
• every run of tests should give the same output.
• Refer to tests of listenbrainz_spark/utils for better understanding.
• A little knowledge of pyspark sql maybe required as you write the tests. Refer to the docs.
• We use pytest for testing. You can refer to the quick guide or the official documentation