Support tests of private ArcGIS Enterprise endpoints

In the current design arcgis-enterprise-base-aws test workflow calls base ArcGIS Enterprise service endpoints from GitHub Actions runner container. This will not work if

- Access to the endpoints is blocked by a firewall, or
- The endpoint is not accessible from the internet at all.

The test should run in a VM or container within the VPC subnets used by the application load balancer. In particular, the tests could use AWS ECS or AWS Lambda (serverless) services.