Adopt helm-unittest for chart template testing

[germangarces]

Problem

All chart testing relies on ct lint + ct install with CI values files. Each file triggers a full Helm install into a kind cluster, so adding a file per feature permutation compounds CI time.

Proposal

Adopt helm-unittest for fast, template-level testing alongside the existing CI setup:

• helm-unittest: fast YAML-based tests for value permutations, conditionals, and edge cases (no cluster needed, runs in seconds)
• ct install: keep a small set of CI values files for happy-path integration tests only

This hybrid approach is used by ingress-nginx and prometheus-community.