Queue memoization for test suite

Setting up the queue takes time, and the test suite is already slow -- it would be a big advantage not to have to execute get_queue every time.

We can't do a one-time memoization, as the ctx_factory can vary. I wonder if we could use ctx_factory.__str__(), something like this:

_QUEUES = {}

def get_queue(ctx_factory):
    id = ctx_factory.__str__()
    if not id in _QUEUES:
        add_queue(ctx_factory)
    return _QUEUES[id]

def add_queue(ctx_factory):
    id = ctx_factory.__str__()
    ctx = ctx_factory()
    _QUEUES[id] = cl.CommandQueue(ctx)