Add script and CI workflow for helm-unittest

- Adds script to run helm-unittest locally
- Adds CI workflow to run helm-unittest in GitHub Actions

.github/workflows/helm-unittest.yaml

@@ -0,0 +1,20 @@
+# https://github.com/marketplace/actions/helm-unit-tests
+
+name: Run Helm unit tests
+
+on: pull_request
+
+# Do not grant jobs any permissions by default
+permissions: {}
+
+jobs:
+  unittest:
+    runs-on: ubuntu-latest
+    permissions:
+      # required to read from the repo
+      contents: read
+    steps:
+      - uses: actions/checkout@v3
+      - uses: d3adb5/helm-unittest-action@v2
+        with:
+          helm-version: v3.15.3

scripts/helm_unittest.sh

@@ -0,0 +1,21 @@
+#!/usr/bin/env bash
+
+# This script uses https://github.com/helm-unittest/helm-unittest
+# to run unit tests for our Helm Chart templates.
+#
+# It uses the Docker image to make it easier to run on local machines without
+# having to manage the binary and its correct version. Note that if it ever
+# appears in https://mise.jdx.dev/registry.html, we can add an entry in .mise.toml.
+#
+# Dependencies:
+# - docker
+#
+# Usage:
+#  ./scripts/helm_unittest.sh
+
+version=${VERSION:-3.15.3-0.5.2}
+
+docker run \
+  -it --rm \
+  -v $(pwd):/apps \
+  helmunittest/helm-unittest:${version} charts/prefect-server