The `brew test` tests were failing as they were unable to include
`test/unit/assertions`. This is because it's a gem and we were setting
the `GEM_HOME` so system gems were being ignored.
While I was there and examining the `$LOAD_PATH`: reduce the number of
things we add there by using `Bundler.with_clean_env` and only adding
`HOMEBREW_LIBRARY_PATH` in the `$LOAD_PATH` if it isn't already there
(which it always is [and has to be] for integration tests).
This also seems to have the side effect of speeding up integration
tests from 1m26s to 1m8s on my machine.
cb3a47d9e2