[Docs] Improve testing section of the pulp developer docs
Feedback from Gerrod:
For testing it would be helpful to mention the fixtures and building local fixtures with pfixtures if a plugin uses them. It would be helpful to explicitly mention that you should use prestart before testing any changes. Also, running Pulp in the foreground is mentioned, but I think you should also mention pstop and individually stopping and starting the different parts of Pulp like the content server, since these parts are useful to test/debug on their own.