systemtest: compare fields instead of _source in approvals

[axw]

Motivation/summary

This is a precursor to #12066. We flatten test approvals because with the move to the apm-data plugin, some fields will be moving from _source to index-time runtime fields.

Note to reviewer: most of the changes come from 749c03e, which is entirely mechanical. The interesting bits are in the other commits.

Checklist

- [ ] Update CHANGELOG.asciidoc
- [ ] Update package changelog.yml (only if changes to apmpackage have been made)
- [ ] Documentation has been updated

How to test these changes

N/A, non-functional change.

Related issues

Closes #11240

[mergify]

This pull request does not have a backport label. Could you fix it @axw? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.17 is the label to automatically backport to the 7.17 branch.
• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.

NOTE: backport-skip has been added to this pull request.

[mergify]

This pull request is now in conflicts. Could you fix it @axw? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b approvefields upstream/approvefields
git merge upstream/main
git push upstream approvefields