Add, or resolve, option to build exclude MPI-depndent tests

[ireed]

Compilation is dependent on having MPI. This requirement unnecessarily prevents the Mellanox tests from being standalone, despite only ~3 out of ~140 tests using MPI. Since only 1 test suite requires MPI (mix), there could be an option for building all tests except that specific suite.
I see Makefile options for 'make all' and 'make oshmem_test'. I presume the latter may have been intended to handle this case (hence the "or resolve" in the title)?