tests

Change a bit the interface to ensure that matrix structure cannot be
modified once the matrix is built.

Actually the interface of the underlying `Kokkos::StaticCrsGraph` is
quite crappy, especially to prevent matrix changes.

Replacing `Kokkos::StaticCrsGraph` class by an appropriate builtin one
should be considered: one could then provide a better interface, and
offer a better coupling with external linear algebra.  Also, this
would reduce the direct dependency on Kokkos.