Use test utils in test_suspend_flow

Use tasks from taskflow.tests.utils in test_suspend_flow instead
of having an outdated copy of them.

Change-Id: I53fad3ffd132830f806836af663ebe3804d47131
Use listener instead of AutoSuspendTask in test_suspend_flow

Instead of injecting engine into storage (which is not clean, not
generally possible, and is an ugly hack) tests for suspension and
resumption now use SuspendingListener which suspends engine
when task with given name reaches given state. This gives us
cleaner and more portable across engines and persistence backends
solution for testing suspension.

Change-Id: Ieca80994bf4e2b6ee3a6249b644365136858bdd0
Rename self._zk to self._client

Make the name consistent between ZkBackend and ZkConnection.

Change-Id: Ie2fe62cc1cb65764f0699de8038f34d83a137086
Merge "Use test utils in test_suspend_flow"
Merge "Use listener instead of AutoSuspendTask in test_suspend_flow"
Merge "Rename self._zk to self._client"