PySpark SQL's `readerwriter` and `window` doctests weren't being run by our test runner script; this patch re-enables them.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #6542 from JoshRosen/enable-more-pyspark-sql-tests and squashes the following commits:

9f46ce4 [Josh Rosen] Enable PySpark SQL readerwriter and window tests.