Add a unit test to clarify ST_ClosestPoint

It was ambiguous which point was returned by ST_ClosestPoint when there
were infinitely many closest points. This unit test shows that the point
returned by ST_ClosestPoint depends on the orientations of geometries A
and B. If they have the same orientation, the point returned is the
first point found in A. If they have opposite orientation, the point
returned is the point of A closest to the first point found in B.