iox-#2251 Fix 'Listener' examples

[elBoberido]

Notes for Reviewer

The listener examples need to drain the subscriber queue in the callback

Pre-Review Checklist for the PR Author

1. [x] Code follows the coding style of CONTRIBUTING.md
2. [x] Tests follow the best practice for testing
3. [x] Changelog updated in the unreleased section including API breaking changes
4. [x] Branch follows the naming format (iox-123-this-is-a-branch)
5. [x] Commits messages are according to this guideline
   • [x] Commit messages have the issue ID (iox-#123 commit text)
   • [x] Commit author matches Eclipse Contributor Agreement (and ECA is signed)
6. [x] Update the PR title
   • Follow the same conventions as for commit messages
   • Link to the relevant issue
7. [x] Relevant issues are linked
8. [x] Add sensible notes for the reviewer
9. [x] All checks have passed (except task-list-completed)
10. [x] Assign PR to reviewer

Checklist for the PR Reviewer

• [x] Consider a second reviewer for complex new features or larger refactorings
• [x] Commits are properly organized and messages are according to the guideline
• [x] Code according to our coding style and naming conventions
• [x] Unit tests have been written for new behavior
• [x] Public API changes are documented via doxygen
• [x] Copyright owner are updated in the changed files
• [x] All touched (C/C++) source code files from iceoryx_hoofs have been added to ./clang-tidy-diff-scans.txt
• [x] PR title describes the changes

Post-review Checklist for the PR Author

1. [x] All open points are addressed and tracked via issues

References

• Closes #2251