minor #10451 Fix phpunit command (Lreus)

This PR was squashed before being merged into the 4.1 branch (closes #10451).

Discussion
----------

Fix phpunit command

replace missing command  ./bin/phpunit with ./vendor/bin/simple-phpunit

<!--

If your pull request fixes a BUG, use the oldest maintained branch that contains
the bug (see https://symfony.com/roadmap for the list of maintained branches).

If your pull request documents a NEW FEATURE, use the same Symfony branch where
the feature was introduced (and `master` for features of unreleased versions).

-->

Commits
-------

2ad4609 Fix phpunit command

Author: javiereguiluz

testing.rst

@@ -31,6 +31,23 @@ command:
 
     $ ./bin/phpunit
 
+.. note::
+
+    If your project is missing the phpunit command in the ./bin directory, Symfony
+    may have not executed the required PhPUnitBridge recipes. This may occurs when
+    the bundle symfony/phpunit-bridge is part of your composer.json file and your 
+    starting project contains a symfony.lock file.
+    
+    Two options can be used to fix that problem:
+    
+        * Remove the symfony.lock file before doing the first ``composer install``
+        * Remove and require again the symfony/phpunit-bridge bundle:
+    
+.. code-block:: terminal
+
+        $ composer remove symfony/phpunit-bridge
+        $ composer require --dev symfony/phpunit-bridge
+
 PHPUnit is configured by the ``phpunit.xml.dist`` file in the root of your
 Symfony application.