Add spn testing setup

spn/testing/README.md

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+# Testing Port17
+
+## Simple Docker Setup
+
+Run `run.sh` to start the docker compose test network.
+Then, connect to the test network, by starting the core with the "test" spn map and the correct bootstrap file.
+
+Run `stop.sh` to remove all docker resources again.
+
+Setup Guide can be found in the directory.
+
+## Advanced Setup with Shadow
+
+For advanced testing we use [shadow](https://github.com/shadow/shadow).
+The following section will help you set up shadow and will guide you how to test Port17 in a local Shadow environment.
+
+### Setting up
+
+Download the docker version from here: [https://security.cs.georgetown.edu/shadow-docker-images/shadow-standalone.tar.gz](https://security.cs.georgetown.edu/shadow-docker-images/shadow-standalone.tar.gz)
+
+Then import the image into docker with `gunzip -c shadow-standalone.tar.gz | sudo docker load`.
+
+### Running
+
+Execute `sudo docker run -t -i -u shadow shadow-standalone /bin/bash` to start an interactive container with shadow.