Add parallel testing to SOPFLOW toml infrastructure

[bjpalmer]

This issue was originally created because tests of SOPFLOW only run with one MPI rank using the CMake variable ${RUNCMD}.

Some of these tests are now running on 3 ranks.

Asher's original issue specified that the driver tests/functionality/sopflow/selfcheck.cpp should be extened to:

• Pass the number of MPI ranks (MPI_COMM_WORLD_SIZE) to the PflowFunctionalityTests constructor
• Read in n_procs from each testcase
• Only use the testcase iff n_procs == MPI_COMM_WORLD_SIZE
• Correct the number of iterations for each testcase to pass in CI

However, passing the number of MPI ranks to the constructor is a bad way of handling this. A much better way is to pass an MPI communicator to the SOPFLOW constructor. This hasn't been done either, but the SOPFLOWCreate command does take a communicator and uses it to set the communicator of the SOPFLOW object.

[bjpalmer]

@abhyshr, it looks like SOPFLOW has several tests that are running on 3 processes. Can you take a look at the tests/functionality/sopflow/CMakeLists.txt file and see if you are okay with coverage of SOPFLOW on multiple MPI ranks?