SYCL does not support some of the warp functions

[fwyzard]

According to the documentation of the SYCL Group functions 1 and SYCL Group algorithms library 2, calling sycl::all_of_group with some threads having exited the kernel is undefined behaviour. This was confirmed during a support meeting with Intel.

oneAPI has an extension to support non-uniform (sub)groups: sycl_ext_oneapi_non_uniform_groups.
The implementation of the SYCL warp functions should be extended using it; until then, we disable the failing test when using a SYCL back-end and instead print an informational message via #2468 / #2469 .