[8.16](backport #5889) Add integration tests using a proxy with mTLS for control plane with Elastic Defend installed

[mergify]

What does this PR do?

Add integration tests for the Elastic Agent running Elastic Defend with a mTLS proxy
See #5716 for tje covered test cases

Why is it important?

To have automated tests covering mTLS and Elastic Defend

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](https://github.com/elastic/elastic-agent#changelog)
• I have added an integration test or an E2E test

Disruptive User Impact

• N/A

How to test this PR locally

In short, manually reproduce the tests described on #5716.

The long version:

• run 3 proxies:

  • mTLScli -> a proxy with mTLS to be passed as cli arguments
  • mTLSpolicy -> a proxy with mTLS configured through the policy
  • oneWayTLSpolicy -> a proxy with simple/one way TLS which defines its CA in the policy
  • all the certificates must be RSA because of an Elastic Defend restriction

• set up the proxies:

  • add a test to testing/integration/endpoint_security_test.go to configure and run the needed proxies:

func TestDebugTestProxies(t *testing.T) {
	prepareProxies(t, &url.URL{Host: "a.wrong.fleet.host"}, "the.real.fleet.host")

	t.Logf("CTRL + C to exit")
	<-make(chan struct{})
}

• run the test with:

go test -v -timeout 0 -tags integration -run TestDebugTestProxies ./testing/integration/

=== RUN   TestDebugTestProxies
    endpoint_security_test.go:1333: [mtlsCLI] certificates saved on: /tmp/TestDebugTestProxies-mtlsCLI2475114678
    endpoint_security_test.go:1333: [mtlsPolicy] certificates saved on: /tmp/TestDebugTestProxies-mtlsPolicy1317235685
    endpoint_security_test.go:1333: [oneWayTLSPolicy] certificates saved on: /tmp/TestDebugTestProxies-oneWayTLSPolicy327431283

• check the logs like [%s] certificates saved on: %s. they will show where the certificates are.
• set up the necessary fleet hosts and proxies on fleetUI
• install/enroll the agent using the configured fleet hosts and proxies. Again, check the test cases on Add an integration test using a proxy with mTLS for control plane and Elastic Defend installed #5716

Related issues

• Closes Add an integration test for --elastic-agent-cert-key-passphrase with Elastc Defend installed #5491
• Closes Add an integration test using a proxy with mTLS for control plane and Elastic Defend installed #5716

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 #5889 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of a338543 has failed:

On branch mergify/bp/8.16/pr-5889
Your branch is up to date with 'origin/8.16'.

You are currently cherry-picking commit a3385439b5.
  (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/testing/define/define.go
	modified:   pkg/testing/fixture.go
	modified:   pkg/testing/fixture_install.go
	modified:   pkg/testing/tools/tools.go
	modified:   testing/integration/endpoint_security_test.go
	modified:   testing/integration/endpoint_test_tools.go

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   testing/proxytest/https.go
	both modified:   testing/proxytest/proxytest.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)

[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/8.16/pr-5889 upstream/mergify/bp/8.16/pr-5889
git merge upstream/8.16
git push upstream mergify/bp/8.16/pr-5889

[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