Standard run:

  cd <testdir>
  py.test

Show stdout:

  py.test -s

Run serial tests in parallel python processes (needs xdist):

  py.test -n 4

Run tests in parallel with MPI:

  mpirun -np 3 python -B -m pytest

Debugging random test deadlocks under MPI:

  cd <testdir>
  while true; do git clean -fdx . && mpirun -n 3 xterm -e gdb -ex r -ex q -args python -m pytest -sv; done

See 'man pytest' for how to use pdb, profiling, coverage, etc.
