minor #15304 [Testing] Application testing in Symfony is fun! Let's document the modern features (wouterj)

This PR was merged into the 4.4 branch.

Discussion
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[Testing] Application testing in Symfony is fun! Let's document the modern features

This PR continues the efforts of restructuring the Testing docs:

- Focus more on Arrange, Act, Assert
- Advertise modern testing features (assertions). Somehow, we didn't mention any easy to use assertion in the main guide, but instead explained the complex dom crawler API.
- Reduce some "duplication". There is an astonishing number of ways to fill in and submit a form (we documented at least 4 different APIs in the main guide!). I've reduced this to two: a plain and short one, and a longer one for more complex form submissions.

Todo, preferably in a follow-up PR: combine the `testing/dom_crawler` and `components/dom_crawler` feature documentation into `testing/dom_crawler`.

cc `@Nyholm`

Commits
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4f72e6a [Testing] Restructure Application Tests section

Author: javiereguiluz

_build/redirection_map

@@ -510,3 +510,4 @@
 /messenger/message-recorder messenger/dispatch_after_current_bus
 /components/stopwatch https://github.com/symfony/stopwatch
 /service_container/3.3-di-changes https://symfony.com/doc/3.4/service_container/3.3-di-changes.html
+/testing/functional_tests_assertions /testing#testing-application-assertions