TestUtils module

[willtebbutt]

There's lots of useful testing utilities in test/common.jl. It would be helpful if these were moved inside src into a module called TestUtils (consistent with standard naming conventions), so that others implementing new NodeOps etc can make use of this functionality (e.g. I'm doing this right now, and thinking "I'd really like to be able to run that _eval function that gets used in the TimeDag tests so that I can be sure my op handles the batch_interval correctly....")

[tpgillam]

We should definitely do this.

I think that doing #34 will be dependent on doing this (and associated test refactoring) first.

My rough takeaways from our meeting:

• We should have a generic function that tests a given node, that dispatches on the type of NodeOp
• Generic tests can be implemented for e.g. BinaryNodeOp, for which alignment semantics should apply.
• In all cases we should verify that the necessary interface is implemented.
• In the documentation we should refer to these tests as the "formal requirement" for ensuring that an interface has been implemented.