Fix test to always use appropriate power bases

Previously, the method constructBoundaryDofs would accept scalar bases
together with blocked bit vectors, and would tacitly do
The Right Thing.

This has changed: Nowadays, the basis really has match the blocking
structure of the bit vectors.  This means that we have to introduce
a second type of power basis:  The one that already exists has the
dimension of the embedding space.  That is correct for sampling
initial configurations.  However, the Dirichlet values apply to
corrections, which live in the tangent space.  Therefore, their
'power-order' needs to be the dimension of that, i.e., lower.