symfony: Broken tests in master: Could not scan for classes inside "tests/"

It seems like some commit broke at least the master. If I run ./phpunit it crashes after installing the packages with:

  [RuntimeException]                                                                          
  Could not scan for classes inside "tests/" which does not appear to be a file nor a folder  

I did a bit of digging and the only reference I found was a classmap option in composer.json from the zip file in the .phpunit directory. Sadly I was unable to fix it myself as it keeps overwriting everything.

ping @nicolas-grekas