Refactoring of the SETTLE tests

Refactoring of the SETTLE tests

Current version of tests for CUDA version of SETTLE was a quick
addition to the old tests, with direct comparison of the GPU
implementation with the old original CPU-based implementation.
This commit rearranges the test structure, making it possible
to apply the same set of tests to both implementations. There
is no changes to the tests themselves. Currently, comparison tests
will run twice and will dry-run on CUDA builds without CUDA-
capable devices.

TODO: Add comparison with pre-computed values for coordinates,
      velocities and virial. Remove the CPU vs GPU comparison
      tests.

Refs #2886, #2888.

Change-Id: Ifcb6af9af6c93787b919b785348f9f4547b6c267