Racey test MirSurfaceVisibilityEvent.exposed_received_when_surface_raised_over_occluding_surface

/home/bschaefer/src/class-test/tests/acceptance-tests/test_client_surface_visibility.cpp:179: Failure
Mock function called more times than expected - returning directly.
Function call: handle(0x7fb12c003bd0, 0)
Expected: to be called once
Actual: called twice - over-saturated and active
[ FAILED ] MirSurfaceVisibilityEvent.exposed_received_when_surface_raised_over_occluding_surface (25 ms)

Only fails during make ptest, passes locally.