[MPI-native] Mount libraries to run tests

[csegarragonz]

The way we test the mpi-native implementation in Faabric, only updates the logic within when we bump the version, and create a new release. As a consequence, we won't know if the changes break the mpi-native code until we do so.

However, this is hard to circunvent, as we rely heavily on docker-compose to scale up or down the number of processes we use in the native deployment (bear in mind this whole subproject is aimed for developing and testing purposes, not production environments). Thus, a possible solution would be to mount the updated libraries in the docker-compose file here.

Further, recent PRs like #67 and #65 have increased the number of implemented MPI calls, hence these ought to be included in mpi-native. For a possible implementation, it's always good to see how it's done in faasm.