test: TestMachines.testAddDiskSCSI: fix test hanging on teardown
test: TestMachines.testAddDiskSCSI: fix test hanging on teardown

Closes #14778

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