Include test setup instructions

Author: codingthat

docs/INSTALLATION.md

@@ -11,6 +11,39 @@ After completing the installation, you can verify if the CLI was installed succe
 godot --version
 ```
 
+## Installing the Exercism GDScript track test runner
+
+You'll need [Godot installed][installation] correctly to use the test runner.
+
+To build and test GDScript scripts for Exercism, Godot will be run in "headless" mode from the CLI.
+These instructions currently require Linux and bash.
+
+### Step 1: Installing the overall test runner infrastructure
+
+To set up for testing, clone [https://github.com/exercism/gdscript-test-runner][gdscript-test-runner] and move its contents to `/opt/exercism/gdscript/test-runner/`:
+
+```sh
+git clone https://github.com/exercism/gdscript-test-runner.git
+sudo mv gdscript-test-runner/ /opt/exercism/gdscript/test-runner/
+```
+
+### Step 2: Downloading the single-exercise test runner script
+
+Assuming you have the `exercism` tool set up and have downloaded at least one GDScript exercise, it should have created an `exercism/gdscript` folder to house the exercises.
+Save [the test runner][test-local-gdscript-solution] in this folder and mark the file executable.
+
+## Running tests
+
+With the installation steps done, you should be able start from any exercise directory (e.g., `~/exercism/gdscript/two-fer`) and run the script (living one level up, in `../`) to test your local solution to that exercise:
+
+```sh
+../test-local-gdscript-solution.sh
+```
+
+[installation]: https://exercism.org/docs/tracks/gdscript/installation
+[gdscript-test-runner]: https://github.com/exercism/gdscript-test-runner
+[test-local-gdscript-solution]: https://raw.githubusercontent.com/exercism/gdscript-test-runner/refs/heads/main/bin/test-local-gdscript-solution.sh
+
 ## Using an IDE
 
 For syntax checking of GDScript files from, for example, [Visual Studio Code][vscode], you'll need to keep the Godot GUI running with a project open in order to activate the debug server, and an extension like [godot-tools][godot-tools] installed and activated.