Switch reimplemented runner helper into real TFLint plugin server for testing #64
Description
Currently, we provide TestRunner for plugin developers to test its plugin. This Runner is not a real RPC client, but a dummy that mimics the behavior. The problem with this method is that we need to reimplement the Terraform's configuration loading.
To reduce the hassle of reimplementing, launch the actual TFLint plugin server process, and use it. To achieve this, it is necessary to add features on the TFLint side.