Subsurface tests incorrectly assume that subsurfaces get focus

[AlanGriffiths]

In particular, testing canonical/mir#4483 I see the following:

[  FAILED  ] TouchInputSubsurfaces/SubsurfaceTest.place_below_simple/0
[  FAILED  ] XdgShellStableSubsurfaces/SubsurfaceTest.place_above_simple/0
[  FAILED  ] XdgShellStableSubsurfaces/SubsurfaceTest.place_below_simple/0
[  FAILED  ] XdgShellV6Subsurfaces/SubsurfaceTest.place_above_simple/0
[  FAILED  ] XdgShellV6Subsurfaces/SubsurfaceTest.place_below_simple/0
[  FAILED  ] WlShellSubsurfaces/SubsurfaceTest.place_above_simple/0
[  FAILED  ] TouchInputSubsurfaces/SubsurfaceTest.place_above_simple/0
[  FAILED  ] WlShellSubsurfaces/SubsurfaceTest.place_below_simple/0

This may be because the protocol was under-specified at the time of writing the tests, but has now been clarified: https://gitlab.freedesktop.org/wayland/wayland/-/merge_requests/236