SPIP: PySpark Test Framework

Currently, there's no official PySpark test framework, but only various open-source repos and blog posts. Many of these open-source resources are very popular, which demonstrates user-demand for PySpark testing capabilities. spark-testing-base has 1.4k stars, and chispa has 532k downloads/month. However, it can be confusing for users to piece together disparate resources to write their own PySpark tests (see The Elephant in the Room: How to Write PySpark Tests). We can streamline and simplify the testing process by incorporating test features, such as a PySpark Test Base class (which allows tests to share Spark sessions) and test util functions (for example, asserting dataframe and schema equality). Please see the full SPIP document attached: https://docs.google.com/document/d/1OkyBn3JbEHkkQgSQ45Lq82esXjr9rm2Vj7Ih_4zycRc/edit#heading=h.f5f0u2riv07v.