Replace pip install -e with pip install -r requirements_test_min.txt (#6588)

Co-authored-by: Daniël van Noord <[email protected]>

Author: edreamleo

doc/development_guide/testing.rst

@@ -14,9 +14,11 @@ Pylint is very well tested and has a high code coverage. New contributions are n
 unless they include tests.
 
 Before you start testing your code, you need to install your source-code package locally.
-To set up your environment for testing, open a terminal outside of your forked repository and run:
+Suppose you have cloned pylint into a directory, say ``my-pylint``.
+To set up your environment for testing, open a terminal and run:
 
-      pip install -e <forked_repo_dir_name>
+    cd my-pylint
+    pip install -r requirements_test_min.txt
 
 This ensures your testing environment is similar to Pylint's testing environment on GitHub.