This test makes sure composited layers make layers underneath fall back to
composition too, in a transitive manner. That is, if plane A is under plane B
which is itself under plane C, plane A falling back to composition means that
both planes B and C fall back to composition too (even if C isn't directly
underneath A).
If a layer cannot be mapped to a plane, make it so all layers under it also fail
plane allocation so that they can be properly composited on the primary plane.
A test was incorrect and has been fixed. Another test had two possible solutions
and has been updated with the one libliftoff yields.
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.
