There were a few tests which require core to be tapped and fail
if it isn't. This is annoying if someone is trying to contribute
to the project and they're using the JSON API instead of having
the core repo tapped locally.
I'm just skipping these because it's the simplest thing to do.
The tests that failed are mostly rubocop tests so it's fine
if they only run on CI.
This case is _really_ slow even although it's something we encourage
people to run often and build systems often do. The `brew --prefix`
case is really fast because it's just in Bash so: let's pull the
`brew --prefix <formula>` case into Bash too.
This doesn't handle any edge-cases like `--installed` and the formula
detection is pretty simple.
Also, to make this behaviour consistent, never output `Cellar` paths
from the (Ruby) `brew --prefix`; we never want people relying on the
Cellar paths anyway, only output them if the formula wasn't installed
(where, arguably, using a Cellar path is even worse) and the speedup is
worth this deviation in behaviour.
