[Filebeat/Journald] Fix flakiness from TestDoubleStarCanBeUsed

[belimawr]

Proposed commit message

This commit fixes the flakiness from TestDoubleStarCanBeUsed by
loosening the number of ingested events, now only 11 out of 30 are
required. This is still enough to ensure we're reading from more than
one journal file.

This commit also collects the logs from the journald input in memory
and writes them to a file if the test fails, this file is collected by
BuildKite for further investigation.

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.next.asciidoc or CHANGELOG-developer.next.asciidoc.

## Disruptive User Impact
## Author's Checklist

How to test this PR locally

Run the flaky test a large enough number of times, it should always pass

go test -v -count 1000 -run TestDoubleStarCanBeUsed ./filebeat/input/journald

## Related issues

• Closes [Flaky test] filebeat/input/journald TestDoubleStarCanBeUsed #46904

## Use cases
## Screenshots
## Logs

[github-actions]

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[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @belimawr? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[belimawr]

Some tests failed because the 9.3 snapshot artefacts cannot be found yet.