doc: reword "test directory"

PR-URL: #42817
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Mestery <mestery@protonmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>

BUILDING.md

@@ -339,7 +339,8 @@ by providing the name of a subsystem:
 $ tools/test.py child-process
 ```
 
-You can also execute the tests in a tests directory (such as `test/message`):
+You can also execute the tests in a test suite directory
+(such as `test/message`):
 
 ```text
 $ tools/test.py test/message