test_work_item_functions missing few points

There are few issues in test [test_work_item_functions](https://github.com/KhronosGroup/OpenCL-CTS/blob/main/test_conformance/basic/test_work_item_functions.cpp) I've noticed while working on issue #2005:

1) local work size is always hardcoded to 1 despite it is previously computed.
2) not all thread's result are verified

I think it could be improved by covering below cases:

a) additional test with **uniform** work group size > 1
b) additional test with **non-uniform** work group size > 1 if `CL_DEVICE_NON_UNIFORM_WORK_GROUP_SUPPORT` supported
c) tested all thread's results including correct results of `get_local_size` with non-uniform group size (if supported)

Start with test_conformance/basic/test_work_item_functions.cpp and review how the existing work-group size is computed, how kernel results are collected, and which device capabilities are checked. Run the existing test first, then cover uniform and supported non-uniform work groups greater than one. Done means every thread result, including get_local_size for non-uniform groups, is verified.

Written by the indexing model from the issue text.