Mention that concept exercise tests should be stand alone (#922)

Author: angelikatyborska

.github/pr-commenter.yml

@@ -60,6 +60,7 @@ comment:
         - **Concept exercise tests changed**
             - ⚪️ Are all tests _un-skipped_?
             - 🔢 Are all tests annotated with `@tag task_id`?
+            - 🐈 Can all tests be understood by reading the test's block only (e.g. no module attributes, no setup functions)?
 
     - id: concept-changed
       files:

CONTRIBUTING.md

@@ -71,6 +71,8 @@ Please keep the following in mind:
 
 - No tests should be skipped for concept exercises.
 
+- For concept exercises, each test block should be understandable without having to reference anything outside the test block, e.g. no module attributes, no setup functions. This is because the platform doesn't show the test files for concept exercises in the web editor. Only the single test's block is shown by the Test Runner if the test failed.
+
 - `.meta/tests.toml` files are generated and updated by `configlet`.
 
     ```shell