Add pytest plugin for project-agnostic testing of flowsheet interfaces

[lbianchi-lbl]

• This is still rough around the edges, but the basic functionality described in Develop testing infrastructure to allow project-agnostic testing of flowsheet interfaces #3 is there
• For reviewing this, I'd suggest to try to follow the steps in the README and see if things work as described
• In addition to any bugs you'll find, I also encourage you to provide feedback on the design of the CLI interface and the general workflow, both low-lever (e.g. naming for CLI flags) and high-level (e.g. whether entry points groups and Python modules are a good choice of supported ways to specify where the flowsheet interfaces should be loaded from)
• One thing to keep in mind is that having this merged soon (ideally before next week) would be needed if we want to finish the migration of watertap-org/watertap-ui in time for the December release

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conda create -n test-ifp-6 --yes -c conda-forge python=3.11 && conda activate test-ifp-6
gh pr checkout 6  # or `git checkout` this branch
pip install -r requirements-dev.txt
# then follow the instructions in `idaes_flowsheet_processor/_testing/README.md

[MichaelPesce]

@lbianchi-lbl this looks good to me. The methods for specifying which flowsheets to test both make sense to me and are easy to do following the readme.

I ran the tests using watertap.flowsheets locally on my mac, with everything passing (except for BSM2, which I anticipated as it has been a problem for me on mac for some time now, so not an issue with the test setup). The additional option for modules is nice - I was also able to test this successfully with the prommis uky flowsheet.