OpenMP sub-group sizes > 1

**Describe the motivation for the feature request**
The OpenMP back-end of AdaptiveCpp for `info::device::sub_group_sizes` returns a vector of one element: `{1}`. This makes it quite hard to test sub-group-aware kernels in environments where device accelerators are not available.

**Describe the solution you'd like**
Support some number of power-of-2 sub-group sizes.

**If applicable, describe alternatives you've considered**
Without resorting to a proper SIMT transformation, all sub-group behavior will be emulated, but for testing purposes that should be acceptable.

**Additional context**
We would like to test sub-group-aware code containers and CI in a more meaningful way than currently possible.