Enable connector tests in CI

[EwoutH]

Currently the connector tests (in test/test_connectors/) are disabled in the CI. I think it would be useful to at least test them periodically, to see if the upstream changes don't break anything.

Currently all the connector tests are disabled, it's also possible to enable then one-by-one instead of all at the same time.

Enable connector tests:

• [ ] test_netlogo.py
• [ ] test_pysd_interface.py
• [ ] test_vensim.py
• [ ] test_simio.py (test need to be written)

[quaquel]

You can add test_simio to this list, even if no test for it yet exist.

The bigger question is how we want to approach it. It seems some form of mocking will be required, at least for Vensim, NetLogo and Simio. We could test against pysd directly by including it as a dependency, and likewise with pynetlogy and jpype, but I think that having a standard test setup for connectors is more sensible going forward.