Testing: incorporate Glenn's shell snippet to download the test runner

docs/TESTS.md

@@ -8,13 +8,30 @@ The [test runner][] is used to load and test solutions.
 To run the test runner, download it locally then use `godot` to run the test runner.
 In the future, the test runner will automatically be downloaded alongside exercises when using `exercism download`.
 
+### Fetching the test runner
+
+The following shell snippet can be used to download the test runner.
+
+```bash
+exercism download --track gdscript --exercise hello-world
+cd "$(exercism workspace)/gdscript"
+curl --remote-name https://raw.githubusercontent.com/exercism/gdscript-test-runner/refs/heads/main/bin/test_runner.gd
+```
+
+### Running the test runner
+
+The test runner is run using `godot` in headless mode (no IDE).
+`-s` or `--script` tells Godot to run the test runner script.
+The test runner expects two arguments: the slug and the path to the solution directory.
+
 ```bash
-godot --headless -s /path/to/test_runner.gd -- <solution-slug> <path/to/solution>
+godot --headless -s to/test_runner.gd -- <solution-slug> <path/to/solution>
 ```
 
 For example,
 
 ```bash
+godot --headless -s "$(exercism workspace)/gdscript/test_runner.gd" -- hello-world "$(exercism workspace)/gdscript/hello-world
 ```