Split into individual test suites

Instead of having just two test suites that (actions and reducers) this splits the tests into one test suite per file.

• This is a more common way to structure tests so is (I think) easier to understand
• It's easier to reason about a test suite knowing that it runs in isolation from any other files
• It allows Jest to parallelise test runs (although in the overhead of running each suite individually cancels out the speed benefit in this case)
• It's easier to focus on a single suite with .only

Also

• simplified the way describeReducerSet is called
• simplified the Jest config
• removed the localStorage mock which is no longer needed