minor #20284 [Testing] Update testing.rst (damien-louis)

This PR was submitted for the 7.1 branch but it was merged into the 5.4 branch instead.

Discussion
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[Testing] Update testing.rst

Adding a little info about loginUser() method.

Commits
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9cedefa Update testing.rst

Author: javiereguiluz

testing.rst

@@ -759,6 +759,10 @@ You can pass any
 :class:`Symfony\\Bundle\\FrameworkBundle\\Test\\TestBrowserToken` object and
 stores in the session of the test client.
 
+To set a specific firewall (``main`` is set by default)::
+
+    $client->loginUser($testUser, 'my_firewall');
+
 .. note::
 
     By design, the ``loginUser()`` method doesn't work when using stateless firewalls.