zpos-3x maps three layers to primary, cursor and one of the overlays planes.
zpos-4x-partial sets up 4 layers but is only able to find a plane for 3 layers,
because ordering between the two overlay planes is undefined.
When testing the library, the libdrm dependency is swapped with a mock libdrm.
The test sets up the mock libdrm's internal state and then runs the test.
