[Testing] Restructure Application Tests section

- Focus more on Arrange, Act, Assert
- Advertise modern testing features: application testing in Symfony is fun!
- Move all important application test docs into the main guide

Author: wouterj

_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