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.