Fix make build_tests without optional dependencies
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. I've either fixed or disabled those tests (for those tests that
were testing MPI-only features).
I've also fixed overlappingschawarztest, which did not build when SuperLU was not found.
The commit concerning MPI should probably be backported to the releases/2.7 branch, as it is affected by the same build failures. The fix for overlappingschwarztest only appeared after commit 05f80ffa which fixed a bug which previously disabled the test. Since this commit is not on the releases/2.7 branch, there is probably no need to backport my fix for overlappingschwarztest.