test: Properly clean up dangling sessions
test: Properly clean up dangling sessions

Some tests leave a "State: closing" session behind, which usually still
contains the SSH agent; clean this up more thoroughly with
`kill-session` (like the OSTree branch already does), and insist that
the sessions will actually go away.

This better isolates tests from one another. In particular, it fixes the
frequent check-session failures on arch (which assumes that no Cockpit
session exists initially) after running TestAccounts.testRootLogin
(which tends to leave its session behind).

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