Added test to verify flush on clReleaseCommandQueue with multiple queues

[shajder]

Fixes #2087 according to task description.

[bashbaug]

@kpet can you check that the test description in #2187 is correct?

This test follows the test description exactly (thanks!), but I would like to ensure it is properly testing that the flush happsn on clReleaseCommandQueue and not somewhere else (e.g. the clFinish).

[bashbaug]

Since we've called clFinish on queue_B here, the event_B must already be CL_COMPLETE.

[kpet]

@bashbaug I've reviewed the description and I think I still mostly agree with this. This won't prevent implementations from flushing queue A when clFinish is called on queue B (or elsewhere, they have the freedom) but, importantly, it should never deadlock because of work in queue B that depends on the unflushed work in queue A (because of the prior call to clReleaseCommandQueue on queue A.

Deadlocking is not a great failure mode for a test though so maybe we could call clFlush instead of clFinish on queue B and pool the event's execution status for the command that went to queue B instead.

The following single queue test could be useful to have too:

• Enqueue work in a queue and get an event
• Call clReleaseCommandQueue on that queue
• Poll the execution status of the event, it should transition to CL_COMPLETE "soon".

[lakshmih]

Agree that clFlush on queue B rather than clFinish is more appropriate to prevent the test from deadlocking when the flush on clReleaseCommandQueue is missing.