Subgroups tests - modify default behavior in switch statements

Probably better return "" or assert ?
See comment: https://github.com/KhronosGroup/OpenCL-CTS/pull/1074#discussion_r567874380

Start by reading the discussion linked from issue #1204 in pull request #1074, since no source file, test, or switch statement is identified here. Determine whether the subgroup switch defaults should return an empty string or assert, then confirm that the affected tests cover the chosen behavior.

Written by the indexing model from the issue text.