poetry no longer creates "editable" metadata file, adds work during tests

[Mike Fiedler]

poetry install --dev used to create the package local metadata, similar to python setup.py develop.
This usually created a pytest_socket.egg-info/ directory in the local path, but that's no longer the case.

Thus, during tests, running make test will always run the install target from the Makefile. It's not a long step, but we shouldn't run unnecessary commands if we don't have to.