Remove unhelpful helper usage from a test file - #16644

Thank you for your contribution to Astropy! 🌌 This checklist is meant to remind the package maintainers who will review this pull request of some common things to look for.

• Do the proposed changes actually accomplish desired goals?
• Do the proposed changes follow the Astropy coding guidelines?
• Are tests added/updated as required? If so, do they follow the Astropy testing guidelines?
• Are docs added/updated as required? If so, do they follow the Astropy documentation guidelines?
• Is rebase and/or squash necessary? If so, please provide the author with appropriate instructions. Also see instructions for rebase and squash.
• Did the CI pass? If no, are the failures related? If you need to run daily and weekly cron jobs as part of the PR, please apply the "Extra CI" label. Codestyle issues can be fixed by the bot.
• Is a change log needed? If yes, did the change log check pass? If no, add the "no-changelog-entry-needed" label. If this is a manual backport, use the "skip-changelog-checks" label unless special changelog handling is necessary.
• Is this a big PR that makes a "What's new?" entry worthwhile and if so, is (1) a "what's new" entry included in this PR and (2) the "whatsnew-needed" label applied?
• At the time of adding the milestone, if the milestone set requires a backport to release branch(es), apply the appropriate "backport-X.Y.x" label(s) before merge.

I see lots of open PRs linked to #15782 . Those probably would be outdated after this one is merged and should be closed?

All the other pull requests addressing #15782 seem to be abandoned. If there was any reasonable hope that any of them would be finalized then there would have been no need to open this example pull request.

Thanks, all!

I think we had an extensive discussion especially within #15782 in which cases numpy.testing.assert_equal would provide more helpful diagnostics than assert x == y. Would have been a good opportunity to add these, but we should probably keep a reference to it.

The replacements that are suggested in the issue and applied here are simple and never make the tests worse. It is true that in some cases more extensive edits could improve our tests even more, but the issue has proven to be confusing enough for new contributors (I don't understand why, but empirically it is clear that it is so), therefore it is good to keep instructions and examples simple. More sophisticated follow-up can be done separately.