test_work_item_functions missing few points

[shajder]

There are few issues in test test_work_item_functions 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)