make filestream tests more reliable and provide debug logs

[leehinman]

What does this PR do?

• Adds debug logs to the output of the filestream integration tests if they fail
• replace tight loop with testify.Eventually for waitUntilOffsetInRegistry
• increase prospector check_interval for TestFilestreamTruncateBlockedOutput
• remove unnecessary check for number of events in TestFilestreamTruncateBlockedOutput

Why is it important?

The TestFilestreamTruncateBlockedOutput would fail regularly in CI. These changes stop the failures locally, but even if the failures occur in CI, we will now have more debug data to try and address what is failing.

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.

How to test this PR locally

cd filebeat\inputs\filestream
while go test -count 1 -run TestFilestreamTruncateBlockedOutput -tags=integration; do :;  done

Related issues

• Closes [Filebeat] Error with registry and truncated file #35571

[elasticmachine]

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

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @leehinman? 🙏.
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-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[pierrehilbert]

From my understanding, this one should be close in favor of #35759.
@leehinman could you do the necessary if my understanding is correct?

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b 35571_test_filestream_truncate_blocked_output upstream/35571_test_filestream_truncate_blocked_output
git merge upstream/main
git push upstream 35571_test_filestream_truncate_blocked_output

[leehinman]

#35759 is merged and adopted the improvements suggested here.