title: Symfony: How to correctly switch from phpunit/phpunit to phpunit bridge
body: <p>Using Symfony 4, I have recently come across the message "Adding phpunit/phpunit as a dependency is discouraged." which seems to have been added in <a href="https://github.com/symfony/recipes/pull/293" rel="nofollow noreferrer">this PR</a>. The message says to "Run php bin/phpunit" which then seems to download PHPUnit to the bin/ directory. However, I don't see why it is placed there and not in the vendor/ directory. I can see I can change the path using an ENV variable, but I don't want to do that every time I run my tests. I have tried looking up the documentation around this recipe, but there isn't much to go on.<br>
  Am I mis-understanding something?<br>
  Or is the documentation incomplete/misleading?</p>