[cmake][bugfix] Fix the testsuite for sequential builds

Some tests assume MPI being present, although they are not executed in parallel. This information got lost in the transition to dune_add_test and is restored with this patch. Thanks to @markus.blatt for noticing.

This fixes #13 (closed)

Tested with and without MPI: works for me.

mentioned in commit e1689391

Status changed to merged