ci: add Helm unittest plugin integration

- Install helm unittest plugin in CI workflow
- Configure dynamic secret name for eoapi-notifier
- Update test values to use empty secret name for dynamic assignment

Author: pantierra

charts/eoapi/test-k3s-unittest-values.yaml

@@ -62,7 +62,7 @@ eoapi-notifier:
           channel: pgstac_items_change
           connection:
             existingSecret:
-              name: "eoapi-test-pguser-eoapi"
+              name: ""
               keys:
                 username: "user"
                 password: "password"