Clean up unit-tests

[sondove]

Our current suite of unit-tests have been developed in a pretty ad-hoc fashion over a long period of time.

This ticket can be considered closed with a PR that does the following:

• Refactor the tests into a consistent style and functionality (testing shares in plots or mails should be the same etc)
• An update to CONTRIBUTING.md, ideally with a separate document, outlining the style of testing.