minor symfony#7115 Better explain the most common problems when testing emails (javiereguiluz)

This PR was squashed before being merged into the 2.7 branch (closes symfony#7115).

Discussion
----------

Better explain the most common problems when testing emails

Testing emails is simple but in the past I've suffered lots of problems related to this. In fact, a few minutes ago I fixed a long standing wrong test thanks to this comment about page redirections and collectors: symfony/symfony#20282 (comment)

I propose to create a new mini section to better explain the problems that you may face when testing emails.

Commits
-------

c2e0608 Better explain the most common problems when testing emails

Author: xabbuh

email/testing.rst

@@ -26,10 +26,6 @@ Start with an easy controller action that sends an email::
         return $this->render(...);
     }
 
-.. note::
-
-    Don't forget to enable the profiler as explained in :doc:`/testing/profiling`.
-
 In your functional test, use the ``swiftmailer`` collector on the profiler
 to get information about the messages sent on the previous request::
 
@@ -67,4 +63,22 @@ to get information about the messages sent on the previous request::
         }
     }
 
+Troubleshooting
+---------------
+
+Problem: The collector object is ``null``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The email collector is only available when the profiler is enabled and collects
+information, as explained in :doc:`/testing/profiling`.
+
+Problem: The collector doesn't contain the email
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+If a redirection is performed after sending the email (for example when you send
+an email after a form is processed and before redirecting to another page), make
+sure that the test client doesn't follow the redirects, as explained in
+:doc:`/testing`. Otherwise, the collector will contain the information of the
+redirected page and the email won't be accessible.
+
 .. _`Swift Mailer`: http://swiftmailer.org/