Subgroups tests - change variables changes to more readable

Modify variable names similar to ballot functions introduced in https://github.com/KhronosGroup/OpenCL-CTS/pull/1074/commits/c03c9949d233fcbfa5a803e8c964bbe8b36d9feb

Start by locating the subgroup tests and reviewing the ballot-function changes in commit c03c9949d233fcbfa5a803e8c964bbe8b36d9feb. Compare the variable naming in those areas, identify the corresponding subgroup-test variables, and confirm that the tests still build and pass after the names are made consistent.

Written by the indexing model from the issue text.