Can't run tests without LinkML

[cthoyt]

I installed oak with pip install -e .[tests] but it appears that linkml is a requirement for tests and is not installed. The solution is to add an optional dependency for linkml in the tests group, but I don't know how to do this with poetry.

[cmungall]

I'd recommend using poetry for running the test suite.

Currently the test dependencies (including linkml) are in dev-dependencies. I had assumed this was fairly standard for poetry, but the docs seem to suggest both: https://python-poetry.org/docs/master/managing-dependencies/