Tests/cobuying algorithms

[iainkirkpatrick]

coverage of the getters used in the two applications of the buying algorithm (intents -> plans, and displaying current state of group intents on the client)

interested in thoughts on:

• are equality tests the ideal way to go about this? could try and write tests that aren't specific to the mock data used, seems tricky / fraught though...
• should the mock data be more complex for more thorough testing?

this is s start for now at least, makes it easier to protect / refactor these getters if need be

closes #452

[ahdinosaur]

does this test all the "unhappy paths", all the possible ways to break the algorithm?