Enable (a subset of) test suite to run without IDAES extensions

[lbianchi-lbl]

:warning: This is just a naive thought for the sake of discussion, and might not be worth the effort (or be feasible at all) :warning:

• Currently, our use of get_solver() from WaterTAP means that idaes get-extensions needs to be run before being able to access a suitable solver for running the tests. This causes idaes-pse (which is a requirement for running idaes get-extensions) to be a required dependency for testing
• Following #3, this need might be relaxed, potentially opening the possibility of running the tests without having to rely on IDAES extensions
• e.g. if the sample models used are solvable without the IDAES Ipopt, then we could use the ipopt from an alternative source such as Conda, which would remove the idaes-pse requirement for running the tests (or at least those tests that are pure Pyomo, as opposed to e.g. more integration-type tests using WaterTAP flowsheets)

Questions

• Is this feasible, considering the models being used for (non-WaterTAP) tests in our test suite?
• Is this worth considering? i.e. would it be helpful to ensure our tests can be run with non-IDAES Ipopt implementations, e.g. facilitating an eventual pyomo.contrib inclusion?

[bknueven]

The non-WaterTAP tests are simple models which should be solvable with the Conda ipopt, or any optimization solver supported by Pyomo, e.g., it looks like we could probably install a community version of a commercial MIP solver, or HiGHS.