Document required step before full test suite can be executed (#2466)

Co-authored-by: Franck Nijhof <frenck@frenck.nl>

docs/development_testing.md

@@ -17,6 +17,12 @@ To run our linters, on the full code base, run the following command:
 pre-commit run --all-files
 ```
 
+To run the full test suite, more dependencies are required than what is set up in the devcontainer by default. To install all dependencies, activate the virtual environment and run the command:
+
+```shell
+uv pip install -r requirements_test_all.txt
+```
+
 To start the tests, and run the full test suite, activate the virtual environment and run the command:
 
 ```shell