test(x-pack/filebeat): add unit test for s3-sns-sqs

[thomasklinger1234]

What does this PR do?

Adding unit tests for consuming AWS SQS messages sent via AWS SNS and S3.

Why is it important?

Unit tests were not complete for upstream change elastic#25492.

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.

Author's Checklist

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How to test this PR locally

Related issues

• [Enhancement] extract the correct body if s3 notify via SNS->SQS elastic/beats#25492