Add http response status checks to TestSetLogLevelFleetManaged

[pchila]

What does this PR do?

This PR adds some checks on fleet server http responses in integration test TestSetLogLevelFleetManaged which is known to be flaky.
This change should help with the investigation.

Why is it important?

Checklist

• I have read and understood the pull request guidelines of this project.
• 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

No user impact since this is a change in an elastic-agent integration test

How to test this PR locally

Related issues

• Relates [Flaky Test]: TestSetLogLevelFleetManaged – agent never communicated agent-specific log level "debug" to Fleet #5193

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?
• ...

[mergify]

This pull request does not have a backport label. Could you fix it @pchila? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-./d./d is the label that automatically backports to the 8./d branch. /d is the digit
• 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.

[elasticmachine]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: dda4756

Failed CI Steps

• Unit tests - Windows 2016
• x86_64:sudo: fleet-airgapped-privileged

History

• 💔 Build #17591 failed 5fd0232
• 💔 Build #17561 failed 5aa4d03

cc @pchila

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarQube

[pkoutsovasilis]

created an issue to capture a new flaky test here. Every other failure is already reported thus retrying