test: Clean up class structure in check-packagekit
test: Clean up class structure in check-packagekit

Don't repeat the disabling of rhsmd in TestUpdates, derive from
NoSubManCase instead.

Move the unrelated `supported_backend` from NoSubManCase to
TestAutoUpdates, as that's the only place that needs them.

cockpituous/cockpit,martinpitt/cockpit,garrett/cockpit,martinpitt/cockpit,deryni/cockpit,cockpituous/cockpit,martinpitt/cockpit,deryni/cockpit,cockpit-project/cockpit,mvollmer/cockpit,deryni/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,deryni/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,garrett/cockpit,garrett/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,cockpituous/cockpit,mvollmer/cockpit,martinpitt/cockpit,cockpit-project/cockpit,cockpituous/cockpit,mvollmer/cockpit,deryni/cockpit,cockpituous/cockpit,deryni/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,deryni/cockpit,garrett/cockpit,martinpitt/cockpit,cockpit-project/cockpit,mvollmer/cockpit,garrett/cockpit,moraleslazaro/cockpit