resource scheduler: reschedule deferred deploys at previous insert order

[sanderr]

When a deploy task is moved from its agent queue back to the waiting set (e.g. because a dependency was added to the scheduled work), it should retain its queue insert order so that when it is re-queued later on, it can jump (part of) the queue. This to prevent starvation.

Add a test case.

Related to #8015. It was initially assumed to be already implemented so it was deemed out of scope for the ticket.