=per #15575 lessen timing issue in AtLeastOnce spec

* lower redelivery interval does not change semantics here (no one
  answers anyway), and lessens the changes to be "unlucky" with gc pauses
* was unable to reproduce error with snapshotting in 80 builds on
  jenkins. Brainstormed possible errors but not found yet..
Merge pull request #15582 from ktoso/per-stabilise-redelivery-tests-ktoso

=per #15575 lessen timing issue in AtLeastOnce spec