minor symfony#5564 Improve and simplify the contributing instructions about tests (javiereguiluz)

This PR was squashed before being merged into the 2.3 branch (closes symfony#5564).

Discussion
----------

Improve and simplify the contributing instructions about tests

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

Commits
-------

5a942f8 Improve and simplify the contributing instructions about tests

Author: xabbuh

contributing/code/tests.rst

@@ -3,54 +3,32 @@
 Running Symfony Tests
 =====================
 
-Before submitting a :doc:`patch <patches>` for inclusion, you need to run the
-Symfony test suite to check that you have not broken anything.
+The Symfony project uses a third-party service which automatically runs tests
+for any submitted :doc:`patch <patches>`. If the new code breaks any test,
+the pull request will show an error message with a link to the full error details.
 
-PHPUnit
--------
+In any case, it's a good practice to run tests locally before submitting a
+:doc:`patch <patches>` for inclusion, to check that you have not broken anything.
 
-To run the Symfony test suite, `install PHPUnit`_ 4.2 (or later) first.
+.. _phpunit:
+.. _dependencies_optional:
 
-Dependencies (optional)
------------------------
+Before Running the Tests
+------------------------
 
-To run the entire test suite, including tests that depend on external
-dependencies, Symfony needs to be able to autoload them. By default, they are
-autoloaded from ``vendor/`` under the main root directory (see
-``autoload.php.dist``).
-
-The test suite needs the following third-party libraries:
-
-* Doctrine
-* Swift Mailer
-* Twig
-* Monolog
-
-To install them all, use `Composer`_:
-
-Step 1: :doc:`Install Composer globally </cookbook/composer>`
-
-Step 2: Install vendors.
+To run the Symfony test suite, `install PHPUnit`_ 4.2 (or later) first. Then,
+install the external dependencies used during the tests, such as Doctrine, Twig
+and Monolog. To do so, :doc:`install Composer </cookbook/composer>` and execute
+the following:
 
 .. code-block:: bash
 
     $ composer install
 
-.. note::
-
-    Note that the script takes some time to finish.
+.. _running:
 
-After installation, you can update the vendors to their latest version with
-the follow command:
-
-.. code-block:: bash
-
-    $ composer update
-
-Running
--------
-
-First, update the vendors (see above).
+Running the Tests
+-----------------
 
 Then, run the test suite from the Symfony root directory with the following
 command:
@@ -59,40 +37,17 @@ command:
 
     $ phpunit
 
-The output should display ``OK``. If not, you need to figure out what's going on
-and if the tests are broken because of your modifications.
+The output should display ``OK``. If not, read the reported errors to figure out
+what's going on and if the tests are broken because of the new code.
 
 .. tip::
 
-    If you want to test a single component type its path after the ``phpunit``
-    command, e.g.:
+    The entire Symfony suite can take up to several minutes to complete. If you
+    want to test a single component, type its path after the ``phpunit`` command,
+    e.g.:
 
     .. code-block:: bash
 
         $ phpunit src/Symfony/Component/Finder/
 
-.. tip::
-
-    Run the test suite before applying your modifications to check that they
-    run fine on your configuration.
-
-Code Coverage
--------------
-
-If you add a new feature, you also need to check the code coverage by using
-the ``coverage-html`` option:
-
-.. code-block:: bash
-
-    $ phpunit --coverage-html=cov/
-
-Check the code coverage by opening the generated ``cov/index.html`` page in a
-browser.
-
-.. tip::
-
-    The code coverage only works if you have Xdebug enabled and all
-    dependencies installed.
-
-.. _install PHPUnit: https://phpunit.de/manual/current/en/installation.html
-.. _`Composer`: https://getcomposer.org/
+.. _`install PHPUnit`: https://phpunit.de/manual/current/en/installation.html