[8.15](backport #5346) Improve testing logger

[mergify]

What does this PR do?

Adds a PrettyPrintf method to ObservedLogs to logger.NewTesting

Why is it important?

The testing logger is helpful, however on every failed test it's necessary to manually format the logs before printing. That leads to several tests just printing the logs as they are which isn't the best for reading them and investigating why the test failed.

The new PrettyPrintf method provides an easy way to pretty print the logs

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
• [ ] I have added an entry in ./changelog/fragments using the changelog tool
• [ ] I have added an integration test or an E2E test

Disruptive User Impact

None

How to test this PR locally

run the test TestObservedLogs_PrettyPrintf

Related issues

• Relates Improve watcher and TestWatcher_AgentErrorQuick logs #5345

Questions to ask yourself

• How are we going to support this in production?
• How are we going to measure its adoption?
• How are we going to debug this?
• What are the metrics I should take care of?
• ...

---

This is an automatic backport of pull request #5346 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 0126540 has failed:

On branch mergify/bp/8.15/pr-5346
Your branch is up to date with 'origin/8.15'.

You are currently cherry-picking commit 0126540de0.
  (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:   internal/pkg/agent/application/actions/handlers/handler_action_diagnostics_test.go
	modified:   internal/pkg/agent/application/actions/handlers/handler_action_policy_change_test.go
	modified:   internal/pkg/agent/application/actions/handlers/handler_action_settings_test.go
	modified:   internal/pkg/agent/application/apm_config_modifier_test.go
	modified:   internal/pkg/agent/application/application_test.go
	modified:   internal/pkg/agent/application/coordinator/coordinator_unit_test.go
	modified:   internal/pkg/agent/application/dispatcher/dispatcher_test.go
	modified:   internal/pkg/agent/application/gateway/fleet/fleet_gateway_test.go
	modified:   internal/pkg/agent/application/monitoring/reload/reload_test.go
	modified:   internal/pkg/agent/application/upgrade/artifact/config_test.go
	modified:   internal/pkg/agent/application/upgrade/artifact/download/fs/verifier_test.go
	modified:   internal/pkg/agent/application/upgrade/artifact/download/http/downloader_test.go
	modified:   internal/pkg/agent/application/upgrade/artifact/download/snapshot/downloader_test.go
	modified:   internal/pkg/agent/application/upgrade/artifact/download/verifier_test.go
	modified:   internal/pkg/agent/application/upgrade/marker_watcher_test.go
	modified:   internal/pkg/agent/application/upgrade/rollback_test.go
	modified:   internal/pkg/agent/application/upgrade/step_download_test.go
	modified:   internal/pkg/agent/application/upgrade/step_unpack_test.go
	modified:   internal/pkg/agent/application/upgrade/upgrade_test.go
	modified:   internal/pkg/agent/cmd/enroll_cmd_test.go
	modified:   internal/pkg/agent/cmd/watch_test.go
	modified:   internal/pkg/agent/storage/store/migrations_test.go
	modified:   internal/pkg/fleetapi/client/client_test.go
	new file:   pkg/core/logger/loggertest/logger.go
	new file:   pkg/core/logger/loggertest/logger_test.go
	deleted:    pkg/core/logger/testing.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   internal/pkg/agent/application/upgrade/watcher_test.go
	both modified:   pkg/component/runtime/manager_fake_input_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

[mergify]

This pull request has not been merged yet. Could you please review and merge it @AndersonQ? 🙏

[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/8.15/pr-5346 upstream/mergify/bp/8.15/pr-5346
git merge upstream/8.15
git push upstream mergify/bp/8.15/pr-5346

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube