test: remove pixel-tests checkout from test/run
test: remove pixel-tests checkout from test/run

Make sure we remove the checkout of the reference images from here, as
well.  This will save us some trouble if pillow isn't installed, or if
we're not running the pixel tests at all.

cockpit-project/cockpit,cockpit-project/cockpit,martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,martinpitt/cockpit,martinpitt/cockpit,mvollmer/cockpit,cockpit-project/cockpit,mvollmer/cockpit,mvollmer/cockpit,cockpit-project/cockpit,mvollmer/cockpit,martinpitt/cockpit,mvollmer/cockpit