Test fails if OpenCL CPU driver is missing

[diptorupd]

test_memory_cpu_context fails if no CPU driver is found on the system. The test failure stops conda build.

Instead the test should be optionally skipped if no CPU device exists.

[diptorupd]

Same issue exists in queue_manager tests as well.

[diptorupd]

@PokhodenkoSA is there an easy way to test the fixes we have added?

[diptorupd]

Done in #93