[cmake][dune_add_test] Fix/dune add test cmake guard skip test

If a complicated user defined command was specified, it didn't lead to the dummy executable (that return 77) being run but the user defined command instead. This patch makes sure that the correct command is executed which then marks the test as skipped and not as failed. If a test is a parallel and tested with multiple number of ranks all these test are shown as skipped.

@joe This is a revised version of !405 (merged). The test name for multiple rank test has to be set as usual test-mpi-<ranks> so that we don't create multiple targets with the same name.