Subgroups tests - adjust test non_uniform_broadcast and broadcast_first to use mask for active workitems

[gwawiork]

• Remove variable NR_OF_ACTIVE_WORK_ITEMS
• use masks as a pattern for active workitems (like other ballot functions)

[svenvh]

https://github.com/KhronosGroup/OpenCL-CTS/pull/1680 addresses the first point (removal of NR_OF_ACTIVE_WORK_ITEMS).