minor #12647 Add tip about accessing removed services in the testing doc (mmenozzi, weaverryan)

This PR was submitted for the master branch but it was merged into the 4.4 branch instead (closes #12647).

Discussion
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Add tip about accessing removed services in the testing doc

As requested [here](symfony/symfony#30104 (comment)) this commit will add a tip about accessing removed services through the special test container in the testing docs.

Commits
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f038841 clarifying how to create a public alias for a test
67dd3c1 Add tip about accessing removed services in the testing doc

Author: javiereguiluz

testing.rst

@@ -560,6 +560,56 @@ allows fetching both public and all non-removed private services::
 For a list of services available in your application, use the ``debug:container``
 command.
 
+.. tip::
+
+    If a private service is *never* used in your application (outside of your test), it
+    is *removed* from the container and cannot be accessed as described above. In that
+    case, you can create a public alias in the ``test`` environment and access it
+    via that alias:
+    
+    .. configuration-block::
+
+        .. code-block:: yaml
+
+            # config/services_test.yaml
+            services:
+                # access the service in your test via
+                # self::$container->get('test.App\Test\SomeTestHelper')
+                test.App\Test\SomeTestHelper:
+                    # the id of the private service
+                    alias: 'App\Test\SomeTestHelper'
+                    public: true
+
+        .. code-block:: xml
+
+            <!-- config/services_test.xml -->
+            <?xml version="1.0" encoding="UTF-8" ?>
+            <container xmlns="http://symfony.com/schema/dic/services"
+                xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+                xsi:schemaLocation="http://symfony.com/schema/dic/services
+                    https://symfony.com/schema/dic/services/services-1.0.xsd">
+
+                <services>
+                    <!-- ... -->
+
+                    <service id="test.App\Test\SomeTestHelper" alias="App\Test\SomeTestHelper" public="true"/>
+                </services>
+            </container>
+
+        .. code-block:: php
+
+            // config/services_test.php
+            namespace Symfony\Component\DependencyInjection\Loader\Configurator;
+
+            use App\Service\MessageGenerator;
+            use App\Updates\SiteUpdateManager;
+
+            return function(ContainerConfigurator $configurator) {
+                // ...
+
+                $services->alias('test.App\Test\SomeTestHelper', 'App\Test\SomeTestHelper')->public();
+            };
+
 .. tip::
 
     The special container that gives access to private services exists only in