minor #3592 Added a tip about hardcoding URLs in functional tests (javiereguiluz)

This PR was merged into the 2.3 branch.

Discussion
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Added a tip about hardcoding URLs in functional tests

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all
| Fixed tickets | -

Commits
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b284e88 Fixed a minor typo
220e124 Added a tip about hardcoding URLS in functional tests

Author: weaverryan

book/testing.rst

@@ -320,6 +320,12 @@ into your Symfony2 application::
 The ``request()`` method takes the HTTP method and a URL as arguments and
 returns a ``Crawler`` instance.
 
+.. tip::
+
+    Hardcoding the request URLs is a best practice for functional tests. If the
+    test generates URLs using the Symfony router, it won't detect any change
+    made to the application URLs which may impact the end users.
+
 Use the Crawler to find DOM elements in the Response. These elements can then
 be used to click on links and submit forms::