pubsub: pstest - Support iterating on DeliveryAttempt when dead lettering is enabled

[burdzwastaken]

Is your feature request related to a problem? Please describe.
Enabling dead lettering on pubsub subscription and interacting with it via pstest does not iterate DeliveryAttempt. This makes it hard to test if certain (bad/good) messages will/will not end up in the dead letter queue as they are always currently nil. Here is a code snippet that displays the behaviour we are seeing: https://gist.github.com/burdzwastaken/e3abfc377ea40a4529abf4dca10d12f9

Describe the solution you'd like
We would like pstest to iterate on DeliveryAttempt so that then we could check if bad messages in our tests end up in the dead letter queue after the set value MaxDeliveryAttempts in DeadLetterPolicy is reached.

Describe alternatives you've considered
Currently we are just testing enabling/updating the dead letter queue (which works well!)

[burdzwastaken]

This is probably a duplicate of #5319. I just saw the other issue after working on this one last week.

[burdzwastaken]

#5320