Modify skip condition for tests not involving MPI to include compile-time and runtime check

[oskooi]

Root cause: The test_mpi skip condition used in test_simulation.py (as well as test_dump_load.py) only checks the compile-time flag mp.with_mpi() (was Meep built with MPI?), but contains an assertion which requires a runtime condition (multiple MPI processes via mpirun). When compiled with --with-mpi --with-openmp and run directly (e.g., OMP_NUM_THREADS=2 python -m unittest, i.e., not as part of make check which uses mpirun -np 2 python ...), the test is not skipped but the assertion fails because Get_size() returns 1 (no mpirun).

Fix: Add mp.count_processors() < 2 to the skip condition, so the test is skipped (not failed) when not running mpirun with multiple processes. Also switch from mp.comm.Get_size() to mp.count_processors() for consistency with the C++ API.