Tests interposition

[besnardjb]

Tests

Use SPACK to test this interposition libary on many MPI flavors and versions.

1. Write some simple tests representing expected behavior
2. Combine with spack support to validate on many configurations

Make sure your request matching technique works as it is a tricky point.
See https://tinyurl.com/37bnyu96

[dssgabriel]

Tested with OpenMPI (v4.1.3), MPICH (v4.0.2) and MPC (v4.1.0).
Other MPI implementations that we should try?