[8.10](backport #3378) Refactor pkg/testing and small improvements to testing/integration

[mergify]

This is an automatic backport of pull request #3378 done by Mergify.
Cherry-pick of 2201b6d has failed:

On branch mergify/bp/8.10/pr-3378
Your branch is up to date with 'origin/8.10'.

You are currently cherry-picking commit 2201b6d549.
  (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:   pkg/component/runtime/manager_test.go
	modified:   pkg/testing/fixture_install.go
	deleted:    pkg/testing/tools/agents.go
	modified:   pkg/testing/tools/artifacts_api.go
	modified:   pkg/testing/tools/check/check.go
	renamed:    pkg/testing/tools/elasticsearch.go -> pkg/testing/tools/estools/elasticsearch.go
	modified:   testing/integration/diagnostics_test.go
	modified:   testing/integration/endpoint_security_test.go
	modified:   testing/integration/fqdn_test.go
	modified:   testing/integration/monitoring_logs_test.go
	modified:   testing/integration/proxy_url_test.go

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by them: pkg/testing/tools/cmd.go
	both modified:   pkg/testing/tools/fleettools/fleet.go
	both modified:   pkg/testing/tools/tools.go
	deleted by us:   testing/integration/upgrade_fleet_test.go

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

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[elasticmachine]

💔 Build Failed

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• Start Time: 2023-10-05T13:37:32.221+0000

• Duration: 5 min 47 sec

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• Description: make check-ci

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[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
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 mergify/bp/8.10/pr-3378 upstream/mergify/bp/8.10/pr-3378
git merge upstream/8.10
git push upstream mergify/bp/8.10/pr-3378

[AndersonQ]

there will be no more 8.10.x releases