[8.15](backport #5457) Add integration test upgrading latest release to a build from the current PR

[mergify]

What does this PR do?

Adds an integration test which upgrades the latest Elastic Agent release to the version built form the current PR being tested.

Why is it important?

It'll allow us to catch any bug preventing the agent from upgrading before merging the PR.

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

AGENT_KEEP_INSTALLED=true INSTANCE_PROVISIONER="multipass" SNAPSHOT=true TEST_PLATFORMS="linux/amd64" TEST_PACKAGES="tar.gz" mage integration:single TestFleetPRBuildSelfSigned

Related issues

• Closes Add an upgrade integration test upgrading to the PR build #4560

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?
• ...

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This is an automatic backport of pull request #5457 done by [Mergify](https://mergify.com).

[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