[8.19](backport #47772) otel: fix remaining tests to use in process testing collector

[mergify]

Proposed commit message

This PR is a follow up to #47338 and updates the remaining tests that still used otelbeat to use the in-process OTel testing collector. To avoid cyclic dependency issues, some tests were relocated and parts of the code were moved into a shared package.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability.
• I have added an entry in ./changelog/fragments using the changelog tool.

How to test this PR locally

cd x-pack/filebeat
mage goIntegTest

Related issues

• For https://github.com/elastic/ingest-dev/issues/6207.
• Relates otel: update integration tests to use an in-process testing collector #47338.

  ---

  This is an automatic backport of pull request otel: fix remaining tests to use in process testing collector #47772 done by Mergify.

[mergify]

Cherry-pick of 7577d59 has failed:

On branch mergify/bp/8.19/pr-47772
Your branch is up to date with 'origin/8.19'.

You are currently cherry-picking commit 7577d5964.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   x-pack/filebeat/input/gcppubsub/otel_test.go
	modified:   x-pack/filebeat/input/gcppubsub/pubsub_test.go
	new file:   x-pack/filebeat/input/gcppubsub/testutil/testutil.go
	modified:   x-pack/filebeat/tests/integration/otel_test.go

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by them: x-pack/otel/processor/beatprocessor/integration_test.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[mauri870]

/test