test: Make disable_preload() work on OSTree
test: Make disable_preload() work on OSTree

Use restore_dir() to get a writable directory, so that the method can
create the override file.

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