functionaltests: assert zero APM error logs

[endorama]

Motivation/summary

Functional tests were missing checking APM Server logs to ensure no errors are logged there.

The change has not been applied to test in 8_15_test.go because we are refactoring to use singleUpgradeTestCase.

Checklist

How to test these changes

Run functional-tests workflow.

Related issues

[mergify]

This pull request does not have a backport label. Could you fix it @endorama? 🙏
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.
• backport-9./d is the label to automatically backport to the 9./d branch. /d is the digit.
• backport-8.x is the label to automatically backport to the 8.x branch.
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[endorama]

In draft so I can trigger a run and ensure it's green before involving the team.