BUG: quiverkey must set the vector figure attribute

Without this change, quiverkey would fail at the drawing stage because the figure for the vector PolyCollection was not set.  Bisection indicated that the problem started with b87263, but I haven't figured out why that triggered it; it seems like was an underlying bug in any case.

I added a test to test_quiver, and simplified the two existing tests that were already there.