[wayland] Fix SurfaceInterface::surfaceItemAt
QRectF::contains includes all edges. If two subsurfaces are next to each other ::contains will treat it as having a singularity where a value is on 2 subsurfaces rather than either one of the other, which isn't what we want for our purposes.
The unit test is extended but also featured some wrong values, the biggest clue that it was wrong was that we used different test co-ordiantes for surfaceContains and inputContains.