Improvements to standalone scripts

[awaelchli]

What does this PR do?

Small improvements as a follow up to #11098

In #13841 I introduced the batch size argument for the standalone script, which worked fine for the CI but unfortunately it also broke the following use case, which is handy for locally debugging:

bash run_standalone_tests.sh -k <the_name_of_the_test_case_to_run>

I'm reverting this back to an environment variable because the argument parsing in bash would otherwise get a bit too nasty.

Before submitting

• Was this discussed/approved via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or internal minor changes/refactorings)

PR review

Anyone in the community is free to review the PR once the tests have passed.
Before you start reviewing make sure you have read Review guidelines. In short, see the following bullet-list:

• Is this pull request ready for review? (if not, please submit in draft mode)
• Check that all items from Before submitting are resolved
• Make sure the title is self-explanatory and the description concisely explains the PR
• Add labels and milestones (and optionally projects) to the PR so it can be classified

Did you have fun?

I made sure I had fun coding 🙃

cc @carmocca @akihironitta @Borda @justusschock @awaelchli @rohitgr7

[carmocca]

We use the refactor label for code refactors (this is just a CI change) and the tests label for direct changes to the tests (again, this is just a CI change). Removed them

[carmocca]

#11098 was merged before, but I would still merge the small improvements here.