plotter: parametrize tests with the available engines only
I believe that the previous state of things was broken:
- Plotly tests were skipped if matplotlib was not installed.
- Plotly tests were executed if plotly was not installed.
This should fix the tests to parametrize them with the engines that are present.
It would be nice if people who have plotly could verify that it works. Ideally, someone could check it with plotly but without matplotlib (but is this even possible?).