compat: add test for deserializing stringref enabled and disabled

[yaauie]

Release notes

[rn: skip]

What does this PR do?

Adds tests to prove that enabling the CBOR stringref extension in #17849 does not create a rollback barrier.

Why is it important/What is the impact to the user?

We want to ensure that a user who has begun using LS 9.x (in which we plan to activate the CBOR stringref extension) can safely roll back to LS 8.19, even if stringref-enabled events are left behind in the PQ.

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 (and/or docker env variables)
• I have added tests that prove my fix is effective or that my feature works

Related issues

• Dependency of: pq: activate stringref extension for more-compact PQ representation #17849 (tests must pass on 8.19 without modification of code)

[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

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 27fdc18

[mashhurs]

LGTM!