This tests the periodic boundary conditions which are generated in space_getsid() and used in runner_dopair1_density.
An 8x8x8 mesh is generated that is periodic and 26 pair interactions are computed for the following cells:
- All 8 corner cells
- The cell at the centre of each face
- The cell at the centre of each edge
These checks are performed for both unperturbed and perturbed cells.