Posting this for the users like me. If you want to run a specific test, mw docker mediawiki exec
is the more suitable choice than running Quibble.
Running the tests of a specific extension:
mw dev mw exec -- composer phpunit extensions/Echo/tests/phpunit/
Running a specific test:
mw dev mw exec -- composer phpunit extensions/Echo/tests/phpunit/NotifUserTest.php
See the document for more examples.