Add AveragedFan regression test

It's hard to get something easy to "eyeball norm" in 2D, but the 3D
example seemed okay, and this 2D test runs fast enough to be part of
"make check" and help us catch regressions.
Copy AveragedFan->AveragedTurbine

There won't be too many additional changes, so this might deserve
better refactoring later, but for now we'll just keep the code that
looks correct untouched and see if we can use it to help verify the
new stuff.
Initialize turbine-specific variables