Not sure if I choosed the right project.
Just in case, I apologise.

In https://moodlehq.github.io/moodle-plugin-ci/
in the section: Using in a Local Dev Environment
it is suggested to execute:
php composer.phar create-project moodlehq/moodle-plugin-ci ../moodle-plugin-ci ^4
(that, of course, creates a folder close to the user moodle root named "moodle-plugin-ci")
but just after it is suggested to execute:
../moodlepluginci/bin/moodle-plugin-ci mustache ./mod/forum/
instead of
../moodle-plugin-ci/bin/moodle-plugin-ci mustache ./mod/forum/