feat: singleTest mage target for each integration test package

[pkoutsovasilis]

What does this PR do?

As request in previous PR reviews this PR introduces Single mage targets for the serverless and leak integration test packages, similar to the existing TestKubernetesSingle target. It adds the following functions to the magefile.go:

• TestServerlessSingle
• TestForResourceLeaksSingle

Internally, both new targets delegate to shared helper functions that support selecting a specific test by name. This unifies the way we run individual integration tests across packages and prepares the mage targets for further package splits.

Why is it important?

As part of the effort to split the increasingly large integration test suite into smaller dedicated packages (see parent issue), we also need to ensure that developers can run individual tests from each new package using a consistent interface.

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

None. This PR only introduces new mage targets for developer convenience without affecting existing workflows.

How to test this PR locally

Invoke the new mage targets 🙂

Related issues

• Closes Introduce singleTest mage target for each integration test package #8414
• Relates Split out integration tests into different packages #8087

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarQube

[elasticmachine]

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

[ycombinator]

Please also update https://github.com/elastic/elastic-agent/blob/main/docs/test-framework-dev-guide.md#running-the-tests but maybe we should mention something more generic there and ask developers to rely on mage -l instead of spelling out every single possible integration testing target?