Flaky test: DockerSwarmSuite.TestSwarmRotateUnlockKey

[thaJeztah]

Seen failing on #28221, and #28173 (comment)

@aaronlehmann is working on this, but opening an issue to track #27967 (comment)

04:07:17 ----------------------------------------------------------------------
04:07:17 FAIL: docker_cli_swarm_test.go:952: DockerSwarmSuite.TestSwarmRotateUnlockKey
04:07:17 
04:07:17 [d7e4655c1124a] waiting for daemon to start
04:07:17 [d7e4655c1124a] daemon started
04:07:17 [d7e4655c1124a] exiting daemon
04:07:17 [d7e4655c1124a] waiting for daemon to start
04:07:17 [d7e4655c1124a] daemon started
04:07:17 [d7e4655c1124a] exiting daemon
04:07:17 [d7e4655c1124a] waiting for daemon to start
04:07:17 [d7e4655c1124a] daemon started
04:07:17 docker_cli_swarm_test.go:1020:
04:07:17     c.Assert(err, checker.IsNil, check.Commentf("out: %v", string(out)))
04:07:17 ... value *exec.ExitError = &exec.ExitError{ProcessState:(*os.ProcessState)(0xc4203db880), Stderr:[]uint8(nil)} ("exit status 1")
04:07:17 ... out: Error response from daemon: swarm could not be unlocked: invalid key provided
04:07:17 
04:07:17 
04:07:17 [d7e4655c1124a] exiting daemon
04:07:18 

[aaronlehmann]

I think we've tracked this down to a problem in the test. It was intended to set unlockKey = newUnlockKey at the end of the loop. This is missing, and it prevents a special case from triggering after two iterations through the loop. @cyli is going to include the fix in her vendor PR.