Add a cookbook entry to explain how to clear the cache before tests

cookbook/testing/auto_cache_clear.rst

@@ -0,0 +1,41 @@
+How to automatically clear the cache before tests are run
+=========================================================
+
+Some features need the cache to be cleared in order to work.
+You could have tests failing, because some part of the cache would need to
+be cleared. (For example, new translation resources)
+
+It's a good idea to clear the cache before running your tests, and automate it
+is even better.
+
+Add the following file::
+
+    // app/tests.bootstrap.php
+    if (isset($_ENV['BOOTSTRAP_CLEAR_CACHE_ENV'])) {
+        passthru(sprintf(
+            'php "%s/console" cache:clear --env=%s --no-warmup',
+            __DIR__,
+            $_ENV['BOOTSTRAP_CLEAR_CACHE_ENV']
+        ));
+    }
+
+    require __DIR__.'/bootstrap.php.cache';
+
+Replace the test bootstrap file ``bootstrap.php.cache`` in `app/phpunit.xml` by
+``tests.bootstrap.php``:
+
+.. code-block:: xml
+
+    <!-- app/phpunit.xml -->
+    bootstrap = "tests.bootstrap.php"
+
+And then, you can define in your `phpunit.xml` file which environment you want the
+cache to be cleared:
+
+.. code-block:: xml
+
+    <!-- app/phpunit.xml -->
+    <php>
+        <env name="BOOTSTRAP_CLEAR_CACHE_ENV" value="test"/>
+    </php>
+