Prevent deadlocks in persistable channel pause test (#18410)

Prevent deadlocks in persistable channel pause test (#18410)

* Prevent deadlocks in persistable channel pause test

Because of reuse of the old paused/resumed channels in this test there
was a potential for deadlock. This PR ensures that the channels are always
reobtained.

It further adds some control code to detect hangs in future - and it
ensures that the pausing warning is not shown on shutdown.

Signed-off-by: Andrew Thornton <art27@cantab.net>
* do not warn but do pause

Signed-off-by: Andrew Thornton <art27@cantab.net>