Subgroup testing only uses infinity for cl_half in many cases

The subgroup tests only test with infinity in many cases for cl_half values (i.e. the only bit pattern used is 0x7c00). This compromises the validity of the testing in question. The affects the following tests listed below.

subgroup_functions_ext:
* sub_group_broadcast(half)

subgroup_functions_core:
* sub_group_broadcast(half)

subgroup_functions_non_uniform_vote:
* sub_group_non_uniform_all_equal(half) - use zero 50% of the time, and infinity 50% of the time

subgroup_functions_ballot:
* sub_group_non_uniform_broadcast(half)
* sub_group_broadcast_first(half)

subgroup_functions_shuffle:
* sub_group_shuffle(half)
* sub_group_shuffle_xor(half)

subgroup_functions_shuffle_relative:
* sub_group_shuffle_up(half)
* sub_group_shuffle_down(half)

Start by locating the named subgroup tests: subgroup_functions_ext, subgroup_functions_core, subgroup_functions_non_uniform_vote, subgroup_functions_ballot, subgroup_functions_shuffle, and subgroup_functions_shuffle_relative. Inspect how cl_half values are selected, then update the affected cases so they test beyond the 0x7c00 infinity bit pattern, including the stated zero/infinity split where specified. Done means the listed half subgroup tests exercise varied values rather than only infinity.

Written by the indexing model from the issue text.