Fix launch_testing_ros so it works with pytest 7. (backport #543)

[mergify]

Description

We need this compatibility because Windows pixi is still using an older pytest. This doesn't really
hurt anything either.

While we are in here, fix a pep257 warning that I noticed in the tests.

Is this user-facing behavior change?

No.

Did you use Generative AI?

Yes, Claude Opus 4.7

Additional Information

This is follow-up to #540

This needs to be backported to Lyrical along with #541

---

This is an automatic backport of pull request #543 done by Mergify.

[mergify]

Cherry-pick of 1c0a7ae has failed:

On branch mergify/bp/lyrical/pr-543
Your branch is up to date with 'origin/lyrical'.

You are currently cherry-picking commit 1c0a7ae.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   launch_testing_ros/test/examples/check_msgs_launch_test.py

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   launch_testing_ros/launch_testing_ros_pytest_entrypoint.py

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[clalancette]

This one has a merge conflict because #541 hasn't been merged yet. But because this one is fixing #541 , I've cherry-picked this into that backport instead. So I'm going to close this out in favor of just running CI over there.