minor #6628 Fix for #6625 (kix)

This PR was submitted for the 3.1 branch but it was merged into the 2.7 branch instead (closes #6628).

Discussion
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Fix for #6625

This fix adds a recommendation of installing PHPUnit as a PHAR archive, as suggested in #6625.

Commits
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1f2bbb6 Fix for #6625

Author: wouterj

book/testing.rst

@@ -17,8 +17,9 @@ it has its own excellent `documentation`_.
 
 .. note::
 
-    It's recommended to use the latest stable PHPUnit version (you will have
-    to use version 4.2 or higher to test the Symfony core code itself).
+    It's recommended to use the latest stable PHPUnit version, `installed as
+    PHAR`_ (you will have to use version 4.2 or higher to test the Symfony core 
+    code itself).
 
 Each test - whether it's a unit test or a functional test - is a PHP class
 that should live in the ``Tests/`` subdirectory of your bundles. If you follow
@@ -924,3 +925,4 @@ Learn more
 
 .. _`$_SERVER`: http://php.net/manual/en/reserved.variables.server.php
 .. _`documentation`: https://phpunit.de/manual/current/en/
+.. _`installed as PHAR`: https://phpunit.de/manual/current/en/installation.html#installation.phar