artifacts: exclude unused rubyzip test files from distributions

[yaauie]

Release notes

[rn:skip]

What does this PR do?

Excludes unused testing artifacts from vendored rubyzip dependency, slightly reducing the size and surface-area of distributed artifacts.

Backport targets: 8.0, 7.16

Why is it important/What is the impact to the user?

While the rubyzip testing fixtures are not on the load-path and inaccessible to the logstash process, some of the files they contain are occasionally picked up by automated scans. They are removed upstream in later releases of rubyzip (see: rubyzip/rubyzip#405), but for now we can exclude them from propagating into our distributed artifacts.

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 (and/or docker env variables)
• [ ] I have added tests that prove my fix is effective or that my feature works

How to test this PR locally

1. run any of the assemble*Distribution gradle tasks, such as ./gradlew assembleZipDistribution
2. expand the generated artifact
3. navigate to vendor/bundle/jruby/2.5.0/gems/rubyzip-1.3.0
4. observe no test directory

[andsel]

LGTM