Recommend installing phpunit bridge instead of phpunit

[weaverryan]

See symfony/recipes#292 (comment)

I added the 4.0 milestone, because Flex will show a warning. But really, this should likely be done in 3.3.

[weaverryan]

Also, I believe when you install the bridge with Flex, you get a bin/phpunit shortcut...

@weaverryan Yes, due to the merged pull request at symfony/recipes/pull/293 you now get a bin/phpunit file. I believe that symfony/recipes/pull/292 is now null and void except for the documentation part :-)

My suggestion for the documentation part in testing.rst:

.. note::

    It's recommended to use the latest stable PHPUnit version, `installed as
    PHAR`_.

should also contain a tip:

.. tip::

    If you are using Flex you can simply install the `phpunit-bridge` recipe.
    Using phpunit as a dependency is discouraged.

Let me know your thoughts!

[javiereguiluz]

@weaverryan I prefer to make this change in 4.0 only because if we can assume that Flex is used, then everything is super simple to explain. See #8883.

[javiereguiluz]

Fixed by #8883.