While working on PHPUnit tests for SendGrid extension (see T183572 and https://gerrit.wikimedia.org/r/#/c/404400), it was noticed that phpunit test runner does not load required dependencies specified in composer.json.
I tried looking for similar cases in other extensions tests, but couldn't find anything similar.
It was mentioned by @Addshore on IRC that
The zuul / Jenkins config / job probably needs to be changed to one that does the composer stuff :)
Any tips for best practices for such cases?