Test OpenCL C 1.x memory fences

The `master` branch may test `mem_fence` (I haven't looked into whether the test is doing what I'd expect), and it does not test `read_mem_fence` and `write_mem_fence`.

Obviously these are valid for a 1.2 device, and where never actually removed from OpenCL C so are valid for a 2.x device.

These functions are quite hard to test, so the CTS test may effectively just be testing that they are present.

Start by reviewing the existing mem_fence test on the master branch and checking what behavior it actually exercises. Then determine how read_mem_fence and write_mem_fence can be covered for OpenCL C 1.x and 2.x; done means the CTS includes tests or presence checks for all three functions.

Written by the indexing model from the issue text.