Minor improvements to test configs

Minor improvements to test configs

No functional changes - this just improves the way we specify and
document intent.

Unless there's interaction effects being tested, only one
configuration should document that it is intended to test e.g. "thread
MPI with CUDA".

Made that specific for one such config, and removed the documentation
from a second config, even though it incidentally tests such code
(because it is currently the default).

Added a config that specifically tests thread-MPI (which it was
coincidentally already doing).

Clarified that the test config that covers the SIMD implementation of
search for pair lists to use on the GPU - which SIMD implementation of
the search is tested is merely coincidental, rather than a specific
choice.

Change-Id: Id1889753623f7808cd1bb2fb060ee2b2852dfd94