Working directories are now set seperately from the commands so they
can later be swapped out when running linters over projects is
supported, and also better support filename mapping for running linters
on other machines in future.
With earlier elm versions, a separate package file is maintained for
tests, which when properly configured enabled the compiler to find what
it needed to compile the tests. Under elm 0.19, test dependencies are
managed in the top-level package file, so `elm make` will fail on the
tests. `elm-test make` is required in this case.
See https://github.com/elm-explorations/test/issues/64
