Add more information on test structure

[joshlynchONS]

Add more information on how to structure tests. For example:

• when should use you test classes (what are the benefits)
• test file per module or function
• best way to write tests
• where should your tests file be
• the importance of importing your package
• etc...

[sarahcollyer]

There is an existing subsection on structuring test files that has started to touch on the above. I would recommend taking what has already been written there and expanding it to include the above bullet points, so that it covers structuring your files and the tests themselves. This can then feed in to the subsections on unit tests, integration tests, and end-to-end testing.

DoD: Structuring subsection has been expanded to include both file and test structure