ci: add helm chart-testing and unittest workflow

[giuliocalzo]

Overview

Add a helm GitHub Actions workflow that runs ct lint and helm-unittest against the gpu-operator chart on PRs that touch deployments/, plus a unit-test suite (with snapshots) covering the chart's core templates.

Why helm unittest?

The gpu-operator chart is the primary install surface for the operator and ships a large set of templates whose behavior depends heavily on values.yaml, conditional logic, and platform-specific branches (OpenShift, MIG, NRI/CDI, etc.). Today, regressions in those templates can only be caught by spinning up a cluster, which is slow and not part of every PR.

helm-unittest makes it cheap to lock that behavior down in CI:

• Fast, hermetic feedback. Runs helm template in-process under a few hundred milliseconds, with no cluster, no network, no real Kubernetes API. Suitable for every PR.
• Catches templating regressions early. Snapshot tests on rendered manifests fail loudly on unintended diffs (e.g. accidental image bumps, removed args, changed RBAC verbs, broken priorityClassName handling).
• Documents values.yaml contracts. Each set: block is an executable example of a supported configuration (develMode, imagePullSecrets, custom DCGM/MIG/device-plugin configs, NRI + CDI validation, etc.), so the expected behavior of values is visible in the repo.
• Validates failure paths too. failedTemplate assertions confirm that input validations (e.g. "the NRI Plugin cannot be enabled when CDI is disabled") actually fire, which ct lint alone does not exercise.
• Cheap to maintain. Snapshots regenerate with helm unittest -u, so intentional template changes are an explicit, reviewable diff in the PR.

What's included

• New workflow .github/workflows/helm.yaml with two jobs:
  • lint — helm/chart-testing-action (ct lint) against the chart.
  • unittest — installs helm-unittest and runs helm unittest deployments/gpu-operator.
  • Triggered only on PRs that change deployments/gpu-operator/**.
• New helm-unittest suites under deployments/gpu-operator/tests/ covering: operator deployment, ClusterPolicy, ClusterRole/Binding, Role/Binding, ServiceAccount, NodeFeatureRules, NvidiaDriver, MIG/DCGM/device-plugin custom configs, cleanup and upgrade CRD hooks, OpenShift readonly-fs SCC, extra objects, and input validations. Snapshots are committed under __snapshot__/.

Testing

• helm unittest deployments/gpu-operator passes locally — 17 suites, 62 tests, 24 snapshots.
• CI helm / lint and helm / unittest jobs are expected to be green on this PR.