[Flaky Test] Fix TestHTTPSProxy

[AndersonQ]

What does this PR do?

It moves the close of the proxy and target http server to t.Cleanup()

Why is it important?

Just using defer was causing the last request log to be logged after the test had ended, what lead to a panic.

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

• N/A

How to test this PR locally

On a Mac (the panics happened on Mac)

go test -count 10000 -timeout 0 -run TestHTTPSProxy ./testing/proxytest/

It should succeed

Related issues

• Closes [Flaky Test]: TestHTTPSProxy – data race #5869

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?
• ...

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[pierrehilbert]

From what I understood, it should close the flaky test issue right?
Could you please link the PR to the issue if this is the case?

[AndersonQ]

Could you please link the PR to the issue if this is the case?

My bad, I fixed it

[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