test/verlet: Fix stress tensor initialisation

The full stress tensor calculation introduced in commit:45ce250 missed
out the verlet and verlet_nvt_andersen tests where the stress tensor
needs to be initialised differently to take into account the special
structure of the stress tensor in the gpu implementation.

This fixes #166921.
force_kernel: Do not use texture references in read_stress_tensor*

CUDA 5.0 does not allow references on textures. Instead, pass the
texture<> object by copy.