Bug#972385: petsc4py: autopkgtest failure on amd64: Caught signal number 11 SEGV

[Drew Parsons]

Source: petsc4py
Followup-For: Bug #972385

When the petsc4py tests fail, it seems to correlate with
  mpiexec -n 48 python3 test_mat_cg.py
  
When the tests pass, they pass with
  mpiexec -n 2 python3 test_mat_cg.py
  
  
Evidently the MPI tests are calling on the full number of available
CPUs. The tests are not large enough to run successfully over 48
processes.

In that case we should be able to resolve the problem by constraining
the number of processes used with mpiexec.