test: Don't try to use axe on Firefox
test: Don't try to use axe on Firefox

And if it fails to find axe, write it to stderr and not to stdout.

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