add twin cut-off mdrun test
The twin cut-off (rvdw!=rcoulomb) case needs coverage.
Fix incorrect rvdw on GPU with rvdw<rcoulomb
When rvdw < rcoulomb was set in the mdp file, rvdw would initially
be set to rcoulomb on the GPU. With default mdrun settings,
the correct rvdw would be set after 2*nstlist steps by PME tuning.
TODO: Add an mdrun test case with rvdw<rcoulomb, refs #3062