tests: run 533-randr15.t at the very end
tests: run 533-randr15.t at the very end

The test runs `xrandr setmonitor`, which will otherwise affect any test
scheduled after 533-randr15.t, causing flakyness in t/217-NET_CURRENT_DESKTOP.t
for example.

stapelberg/i3,cornerman/i3,stapelberg/i3,cornerman/i3,Airblader/i3,Airblader/i3-original,Airblader/i3,i3/i3,i3/i3,cornerman/i3,stapelberg/i3,Airblader/i3,Airblader/i3-original,cornerman/i3,i3/i3,Airblader/i3-original,stapelberg/i3,Airblader/i3,i3/i3,Airblader/i3-original,Airblader/i3-original