[9.0] (backport #18124) test: explicitly load ascii fixture as ascii, do line-oriented parsing

[mergify]

Release notes

[rn:skip]

What does this PR do?

Re-works the loading of a text fixture to be less susceptible to encoding-related issues so that they work reliably on both posix systems and windows. The fixtures are lower-ascii, and should be processed in a line-oriented way.

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

N/A

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

Author's Checklist

• observe that the exhaustive tests

How to test this PR locally

Fixes a test that was failing only on Windows, so from a Windows dev environment:

./gradlew :logstash-core:javaTests --tests "org.logstash.EventTest"

Related issues

• Closes [Test Failure] deserializeStringrefExtension are failing on Windows #18122
• Relates pq: activate stringref extension for more-compact PQ representation #17849

Logs

EventTest > deserializeStringrefExtensionEnabled PASSED

-- exhaustive tests - Windows 2025

EventTest > deserializeStringrefExtensionDisabled PASSED

-- exhaustrive tests - Windows 2025

---

This is an automatic backport of pull request #18124 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

[elasticmachine]

💛 Build succeeded, but was flaky

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• Commit: b2f4db1

Failed CI Steps

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cc @yaauie

[yaauie]

✅ Clean and Green 🎉