libgui: Remove custom allocator from tests

The StreamSplitter test used a custom allocator to ensure that only one
GraphicBuffer was ever allocated during the test run. Now we can ensure
that by disabling allocation on the relevant BufferQueues after
allocating that one buffer, so the custom allocator is unnecessary.

Change-Id: I98289bda669c50a93ba9b70ceca1781203ad83b2