Use logp.NewNopLogger for TestGenerateProcessorList

[belimawr]

Proposed commit message

Because logptest.NewTestingLogger uses the testing.T as the output for the logger, it could happen that the processors would log after the test had ended, leading to a panic.

This commit fixes it by using a no-op logger instead.

Example of the test failing: https://buildkite.com/elastic/beats-libbeat/builds/22486/steps/canvas?jid=019a1301-63ab-498b-b294-88e04f93bd9d#019a1301-63ab-498b-b294-88e04f93bd9d/210-617

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. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability.
• I have added an entry in ./changelog/fragments using the changelog tool.

## Disruptive User Impact
## Author's Checklist

How to test this PR locally

cd libbeat/publisher/processing
go test -v -count=1 -run=TestGenerateProcessorList .

## Related issues
## Use cases
## Screenshots
## Logs

[elasticmachine]

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

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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.