Turn on OpenMP for some of the unit tests

[twsearle]

Description

Open MP with 2 processes was failing under the spice intel build. For the time being, this was removed in #51. Once the next generation of SPICE and the HPC have arrived, it would be sensible to turn testing of this back on to double check the MPI code quality.

In the interim, we can avoid using OpenMP without much performance impact.

Acceptance Criteria (Definition of Done)

OpenMP configuration is tested and satisfies some ctests with the same norm calculation output.