[Spike] Investigate Helm `unittest` plugin

Summary

The Runner team is testing the Helm unittest plugin in gitlab-runner!407 (merged). It may be a helpful way to make our tests more accessible to non-Ruby developers.

Acceptance criteria

• Look into unittest in general
• Implement unittest in a small subset of our charts as a proof of concept
• Determine if unittest makes sense to implement for the entire project
• Create follow-up issue(s) as needed