minor symfony#9451 Made explicit testing dependencies (HeahDude, javiereguiluz)

This PR was merged into the 3.4 branch.

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Made explicit testing dependencies

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Commits
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f67a47a Minor reword
a99e2d5 Made explicit testing dependencies

Author: javiereguiluz

testing.rst

@@ -128,6 +128,13 @@ tests as far as PHPUnit is concerned, but they have a very specific workflow:
 * Test the response;
 * Rinse and repeat.
 
+Before creating your first test, install these packages that provide some of the
+utilities used in the functional tests:
+
+.. code-block:: terminal
+
+    $ composer require --dev symfony/browser-kit symfony/css-selector
+
 Your First Functional Test
 ~~~~~~~~~~~~~~~~~~~~~~~~~~