Tests should not depend on external service

The rule curlPost.r relies on an external service, httpbin.org, to be working. While running plugin tests, I got some false negatives that happened because the service appeared to be down/degraded. Our tests probably should not rely on an external service.

https://github.com/irods/irods_microservice_plugins_curl/blob/e50e7dae8cd54e9cb77feab5fef5fdf21a2c3417/rules/curlPost.r#L6