swap -n and -a args in jsrun to ensure tests run smoothly on Summit

# Use jsrun for executing the tests.

set(TEST_RUNNER "jsrun" CACHE STRING "Command for executing (MPI) programs.")

set(MPIEXEC_NUMPROC_FLAG "-a" CACHE STRING

set(MPIEXEC_NUMPROC_FLAG "-n" CACHE STRING

  "Flag used by TEST_RUNNER to specify the number of processes.")

# Use 1 resource set with 1 GPU and 8 cores for executing the tests.

set(MPIEXEC_PREFLAGS "-n 1 -g 1 -c 8" CACHE STRING

# Use 1 resource set with 1 GPU and 5 cores for executing the tests.

set(MPIEXEC_PREFLAGS "-a 1 -g 1 -c 5" CACHE STRING

  "Flags to pass to TEST_RUNNER directly before the executable to run.")

# The flag "--smpiargs=none" is needed to execute tests with no MPI functionalities.

set(SMPIARGS_FLAG_NOMPI "--smpiargs=none" CACHE STRING