Fix DmaAllocator memory leak when closing camera

[will-v-pi]

When closing the camera, the allocator was not closing the open file descriptors, causing a memory leak. Fixed by closing the file descriptors and deleting the allocator when closing the camera.

Fixes #887

[davidplowman]

Looks like it's just timing out. We could either increase the timeout, or shorten the new test a bit!

[will-v-pi]

Probably better to shorten the test - I'll see if configuring a larger config can make the leak occur sooner