Running a php 7.3 build, pre-installed PHPUnit 9 fails:
$ phpunit PHPUnit 9.0.1 by Sebastian Bergmann and contributors. Cannot open file "/home/travis/build/[secure]/carddav2fb/vendor/autoload.php". The command "phpunit" exited with 1.
This happens due to the PHPUnit step executed before the repo is cloned. It seems that PHPUnit 9.1does not have this behaviour.