Restore missing documentation about how to test runtimes standalone.

Author: Jiaxin Fan

tests/src/test/standalone/README.md

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+
+# Tests for OpenWhisk NodeJS Runtime as Standalone Container
+## Building the Runtime Container
+After a runtime container is built, one should be able to run it as a standalone container.
+The following example shows how to generate a Docker image for the Node.js 18 runtime version and test the standalone runtime using the `curl` command. Testing other runtime versions can be done in the same manner.
+
+- Run the `distDocker` command to generate the local Docker image for the desired runtime version.
+```
+./gradlew core:nodejs18Action:distDocker
+```
+This will return the following runtime image with the name `action-nodejs-v18`
+
+## Running the Container
+For the purpose of the test. We are going to access the the runner container via `localhost`.
+The `Action` container exposes `port 8080` (see the Dockerfile for the associated Docker image), thus we publish the container's `port 8080` to the `localhost`:
+```
+docker run --publish 3008:8080 -i -t action-nodejs-v18:latest
+```
+
+## Testing
+This example has prepared a `helloworld.json` file to post using `curl`.
+
+### Initialze the Runtime
+Initialize the runtime with by invoking the ```/init``` endpoint.
+```
+curl -H "Content-Type:application/json" -X POST --data '@openwhisk-runtime-nodejs/tests/src/test/standalone/helloworld/helloworld.json' http://localhost:3008/init
+
+{"OK":true}
+```
+
+#### Run the function
+
+Execute the function using the ```/run``` endpoint.
+
+```
+curl -H ""Content-Type:application/json" -X POST --data '@openwhisk-runtime-nodejs/tests/src/test/standalone/helloworld/helloworld.json' http://localhost:3008/run
+
+{"payload":"Hello World!"}
+```

tests/src/test/standalone/helloworld/helloworld.json

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+{
+  "value": {
+    "name" : "nodejs-helloworld",
+    "main" : "main",
+    "binary": false,
+    "code" : "function main() {return {payload: 'Hello World!'};}"
+  }
+}