Combine Pytest analysis classes with basic Python analysis

[khatchad]

Combine Pytest analysis with basic Python analysis. In other words, merge classes like com.ibm.wala.cast.python.loader.PytestLoader, com.ibm.wala.cast.python.loader.PytestLoaderFactory, and com.ibm.wala.cast.python.client.PytestAnalysisEngine<T> with the general Python analysis classes.