code health: queue tests improvments

Non-critical tests improvements for queue subpackage. It does not affect
a logic of the tests:

* A duplicate code that creates a queue is extracted into a separate
  function.
* A duplicate code that drops a queue is extraced into a separate
  function.
* `t.Errorf() + return` replaced by `t.Fatalf()`.
* `err.Error()` replaced by just `err` in formatted messages.

Author: oleg-jukovec