[Testing] Mention Panther for E2E tests

Author: alexandre-daubois

testing.rst

@@ -1137,12 +1137,30 @@ Mailer Assertions
     address. This assertion normalizes addresses like ``Jane Smith
     <[email protected]>`` into ``[email protected]``.
 
-.. TODO
-..  End to End Tests (E2E)
-..  ----------------------
-..  * panther
-..  * testing javascript
-..  * UX or form collections as example?
+End to End Tests (E2E)
+~~~~~~~~~~~~~~~~~~~~~~
+
+End to end tests (E2E) are a special type of application tests that
+simulate a real user interacting with your application. They are
+typically used to test the user experience (UX) of your application.
+The difference with functional tests detailed above is that E2E tests
+use a real browser instead of a simulated one. This browser can run in
+headless mode (without a graphical interface) or not. The first option
+is convenient for running tests in a Continuous Integration (CI), while
+the second one is useful for debugging purpose.
+
+This is the purpose of `Panther`_, a library that provides a real browser
+to run your tests. Here are a few things that make Panther special, compared
+to other End to End testing tools:
+
+* Possibility to take screenshots of the browser at any time during the test
+* The JavaScript code contained in webpages is executed
+* Panther supports everything that Chrome (or Firefox) implements
+* Convenient way to test real-time applications (e.g. WebSockets, Server-Sent Events
+  with Mercure, etc.)
+
+You can learn more about Panther and how to use it by visiting the
+`Github page of Panther`_, where you'll also find its detailed documentation.
 
 Learn more
 ----------
@@ -1166,3 +1184,5 @@ Learn more
 .. _`SymfonyMakerBundle`: https://symfony.com/doc/current/bundles/SymfonyMakerBundle/index.html
 .. _`PHPUnit Assertion`: https://docs.phpunit.de/en/9.6/assertions.html
 .. _`section 4.1.18 of RFC 3875`: https://tools.ietf.org/html/rfc3875#section-4.1.18
+.. _`Panther`: https://github.com/symfony/panther
+.. _`Github page of Panther`: https://github.com/symfony/panther