doc: enrich test commands with executable

BUILDING.md

@@ -318,36 +318,41 @@ If you are updating tests and want to run tests in a single test file
 (e.g. `test/parallel/test-stream2-transform.js`):
 
 ```text
-$ tools/test.py test/parallel/test-stream2-transform.js
+$ python3 tools/test.py test/parallel/test-stream2-transform.js
 ```
 
+> Note: Using `pyhton3` alias is necessary only if you don't have
+> a python executable on your path.
+
 You can execute the entire suite of tests for a given subsystem
 by providing the name of a subsystem:
 
 ```text
-$ tools/test.py child-process
+$ python3 tools/test.py child-process
 ```
 
 You can also execute the tests in a test suite directory
 (such as `test/message`):
 
 ```text
-$ tools/test.py test/message
+$ python3 tools/test.py test/message
 ```
 
 If you want to check the other options, please refer to the help by using
 the `--help` option:
 
 ```text
-$ tools/test.py --help
+$ python3 tools/test.py --help
 ```
 
 You can usually run tests directly with node:
 
 ```text
-$ ./node ./test/parallel/test-stream2-transform.js
+$ ./node test/parallel/test-stream2-transform.js
 ```
 
+> Info: `./node` points to your local Node.js build.
+
 Remember to recompile with `make -j4` in between test runs if you change code in
 the `lib` or `src` directories.