Fix CI build on integration branches

[Mpdreamz]

It appears that our builds are red on integration branches (main/8.0/8.1/8.2/7.17) for a while now:

It's typically one or all of these three tasks:

We should perhaps run less and address anything that's flaky and/or depending on snapshots that could cause a difference in builds/tests after merging.

[axw]

• apm-ci/branch is the pipeline, it'll fail if one of the other stages fail
• beats-tester/branch is known to be failing (beats-tester jobs are failing for beats and apm-server due to changes in output.file handling in beats beats-tester#211), haven't heard any update on this recently but the plan is to migrate to e2e-testing for smoke tests
• system tests are currently plagued by a handful of flaky tests, which we're already tracking

[axw]

Just chatted with @Mpdreamz, we should just disable beats-tester if we're moving to e2e-testing and we don't intend to invest in fixing beats-tester.

[simitt]

Created #7543 for transitioning beats-tester to e2e testing once the automation team's setup is ready.

[simitt]

@axw you mentioned removing the beats-tester from our CI; please use this issue for tracking it.