While we're here, also add `brew tests --no-parallel` which I relied
on during testing.
Pretty much anywhere we rely on a stubbed formula on disk to work: we
need to disable the API.
The extra module was their to facilitate testing but now that
everything is properly namespaced and each command is an instance
we can just move the methods into the command. Since it's an
instance now we don't need to be as careful about caching either.
