make tests succeed when MPI is disabled

When MPI is not available or explicitly disabled with the CMake build option -DCMAKE_IDSABLE_FIND_PACKAGE_MPI=TRUE, some tests were unable to build.

The tests created from solverfactorytest.cc.in and part of scalarproductstest.cc use Dune::OwnerOverlapCopyCommunication which is defined behind #if HAVE_MPI and is thus not available in a non-MPI build. I've thus disabled those tests when MPI is unavailable.

The matrixmarkettest did not work without MPI, as it contained some code using the wrong template parameters when HAVE_MPI was not set. Those template paramters have been fixed now.

I've confirmed, that after my changes make build_tests succeeds to build all tests and that those tests run without failure.