[!743] [test][mpi] run mpipacktest only if MPI was found

Merge branch 'run-mpipacktest-only-with-mpi' into 'master'

ref:core/dune-common @gruenich can we merge this to 2.7?

See merge request core/dune-common!743

(cherry picked from commit 8d50cec3)

691337f3 [test][mpi] run mpipacktest only if MPI was found