Adds a PetscMatrixAssemblyTest
We already individually test for some of the operators that they can be assembled into a PetscSparseMatrix. This new test does this explicitely for a wide range of operators/operator families, i.e. we it does not test all P1ConstantOperators e.g. but only one of them.
Note that operators for which an assembly does currently not work are only reported, but not tested, so the test succeeds currently.
The commit places the test together with PetscTest.cpp into a new Petsc sub-directory.