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.
This probably has to wait until 2.7.0 now and will require a bunch of
formula changes/deprecations but we should probably start moving in this
direction given we're not installing any of these on our CI any more.
