Yet more test refactoring

Note this is a continuation of pubsweet/pubsweet-client!80 (merged) but is split into a separate commit for easier reviewing. Once that is merged, I'll rebase and update the merge target.

• Removing describeReducer results in a net reduction in code and an improvement in readability.

Also:

• Remove unused test callbacks.
• Make all action arguments values instead of functions.
• Rename some variables for clarity.
• Don't test for implementation details like presence of module default export.
• Remove describeReducerSet in favour of individual tests.