Revert "Use 1.5.5 lambda layer for tests because 1.5.6 seems to be broken

[JonasKunz]

We should switch back to testing with latest as soon as the fixed lambda layer has been released:
elastic/apm-aws-lambda#510

When that change has been released, we should be able to simply rerun the tests on this PR and they should become green again.

What does this PR do?

Checklist

• This is an enhancement of existing features, or a new feature in existing plugins
  • I have updated CHANGELOG.asciidoc
  • I have added tests that prove my fix is effective or that my feature works
  • Added an API method or config option? Document in which version this will be introduced
  • I have made corresponding changes to the documentation
• This is a bugfix
  • I have updated CHANGELOG.asciidoc
  • I have added tests that would fail without this fix
• This is a new plugin
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  • My code follows the style guidelines of this project
  • I have made corresponding changes to the documentation
  • I have added tests that prove my fix is effective or that my feature works
  • New and existing unit tests pass locally with my changes
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  • Added an API method or config option? Document in which version this will be introduced
  • Added an instrumentation plugin? Describe how you made sure that old, non-supported versions are not instrumented by accident.
• This is something else
  • I have updated CHANGELOG.asciidoc