`JobIteration::TestHelper` should be refactored

[sambostock]

• It's API is awkward by requiring an object responding to size instead of just taking in an Integer, despite the documentation saying it wants an Integer.
• It stubs out the adapter globally, which probably won't work for per-job class adapters.
• It requires adapter mutability, instead of using a smarter adapter that can be forced on or off.
• It has an implicit dependency on mocha.