feature #14731 [Testing] Restructure/rewrite testing docs (wouterj, Nyholm)

This PR was merged into the 4.4 branch.

Discussion
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[Testing] Restructure/rewrite testing docs

These are my first ideas for a better testing documentation. One of the major results of the Testing survey (and Testing initiative of @Nyholm, @weaverryan and @kbond) was a lack of good documentation beyond the basic stuff.

This is a very very rough sketch of my ideas. I've mostly moved sections around and added some titles. So the actual content probably no longer reads as one article, which is going to be fixed before marking this PR as finished.

Commits
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1091d13 Finish testing article rewrites
42cb887 Finalize testing document rewrite
1380bd7 Very rough sketch of better testing docs

Author: wouterj

.doctor-rst.yaml

@@ -95,3 +95,4 @@ whitelist:
         - ".. _`Deploying Symfony 4 Apps on Heroku`: https://devcenter.heroku.com/articles/deploying-symfony4"
         - '.. versionadded:: 0.2' # MercureBundle
         - '.. code-block:: twig'
+        - 'End to End Tests (E2E)'

best_practices.rst

@@ -399,8 +399,8 @@ Smoke Test your URLs
 
 In software engineering, `smoke testing`_ consists of *"preliminary testing to
 reveal simple failures severe enough to reject a prospective software release"*.
-Using :ref:`PHPUnit data providers <testing-data-providers>` you can define a
-functional test that checks that all application URLs load successfully::
+Using `PHPUnit data providers`_ you can define a functional test that
+checks that all application URLs load successfully::
 
     // tests/ApplicationAvailabilityFunctionalTest.php
     namespace App\Tests;
@@ -454,3 +454,4 @@ you must set up a redirection.
 .. _`feature toggles`: https://en.wikipedia.org/wiki/Feature_toggle
 .. _`smoke testing`: https://en.wikipedia.org/wiki/Smoke_testing_(software)
 .. _`Webpack`: https://webpack.js.org/
+.. _`PHPUnit data providers`: https://phpunit.readthedocs.io/en/stable/writing-tests-for-phpunit.html#data-providers

form/unit_testing.rst

@@ -121,8 +121,8 @@ variable exists and will be available in your form themes::
 
 .. tip::
 
-    Use :ref:`PHPUnit data providers <testing-data-providers>` to test multiple
-    form conditions using the same test code.
+    Use `PHPUnit data providers`_ to test multiple form conditions using
+    the same test code.
 
 .. caution::
 
@@ -242,3 +242,5 @@ guessers using the :method:`Symfony\\Component\\Form\\Test\\FormIntegrationTestC
 :method:`Symfony\\Component\\Form\\Test\\FormIntegrationTestCase::getTypeExtensions`
 and :method:`Symfony\\Component\\Form\\Test\\FormIntegrationTestCase::getTypeGuessers`
 methods.
+
+.. _`PHPUnit data providers`: https://phpunit.readthedocs.io/en/stable/writing-tests-for-phpunit.html#data-providers