Test shapefunction duality

Previously, the testLocalInterpolation method constructed random linear combinations of the shape functions, and then checked whether the 'interpolate' method would recover the random coefficients. This patch rewrites the test to call 'interpolate' on the individual shape functions, and check whether the resulting coefficient set is the correct unit vector. While this does not really result in an increase of test coverage, it puts the test more in line with the classic Ciarlet definition of a (local) finite element. There, the shape functions are defined as the dual basis to the set of degrees of freedom, and the new test tests this directly. It also is a bit shorter, too.