Uniter test cleanup

This started out as a simple move of the metrics code/tests into their own file, and then grew to include more significant test rearrangements:

  * moving all the context/stepper bits into util_test.go
  * moving all the global uniterTest arrays inside the methods that use them
  * dropping uniter_old_test.go, because there's no benefit to keeping its one test method outside uniter_test.go
  * moving UniterSuite.APILogin to context.apiLogin because it seemed cleaner

It's basically all mechanical, though.

(Review request: http://reviews.vapour.ws/r/610/)