Add functional test chapter to mailer

[famoser]

Formulation largely from https://symfony.com/doc/4.4/email.html#how-to-test-that-an-email-is-sent-in-a-functional-test
Simple example which showcases the "global" methods like assertEmailCount and the e-mail specific assertions.
Taken from actual code in use, so should be close to what user wants to accomplish.

Targets the 4.4 branch as described in the docs.
Resolves #14397

[famoser]

The failing build is due to failing:

git clone -b 4.4 --depth 5 --single-branch https://github.com/symfony-tools/symfony-application.git _sf_app
cd _sf_app/
composer update

The problem is that composer update installs symfony@6.0 but then the deprecated Symfony\Component\Routing\RouteCollectionBuilder is still used in src/Kernel.php. Note that https://github.com/symfony-tools/symfony-application/blob/4.4/composer.json uses wildcards for symfony dependencies.

[famoser]

This can be solved either here or upstream.

Here

composer update --no-scripts # prevent the failing cache:clear command to run
composer recipes:install symfony/framework-bundle --force # path Kernel.php
rm -rf vendor/ # remove deps again
composer update # now proceed as before

(note: not removing the vendor directory downgrades the dependencies back to ^4.4. Not seeing why though at the moment.)

Upstream

I think it would be cleaner to solve it upstream: Specify the symfony dependency in composer json: https://github.com/symfony-tools/symfony-application/blob/4.4/composer.json. For example, specify "symfony/framework-bundle": "^4.4" on branch 4.4. I'll ping the author @Nyholm who probably knows why it is not done that way at the moment.

[Nyholm]

You are correct. Branch 4.4 on https://github.com/symfony-tools/symfony-application should install only version 4.4 of all symfony packages. Feel free to prepare a PR and I'll make sure to fix this ASAP.

[Nyholm]

Could we rerun the CI?

Either docs team restart it or @famoser runs:

git commit --allow-empty  -m "Trigger CI" 
git push

[famoser]

Done, thanks for your help!

I see that all PR from the last three days look stuck from the same issue we have just fixed. It might be quicker if the doc team could restart for all of them, rather than each one makes its own empty commit.

[Nyholm]

I've notified the doc team.

Thank you for testing and confirming that it works.

[Nyholm]

I like this PR. Well done!

There is just some syntax fixes.

PS. I know that if we asks the docs team nicely they will do syntax fixes for us.

[Nyholm]

Awesome!
Thank you @famoser for making this PR and to debugging and fixing the CI.

[OskarStark]

Thank you very much, I appreciate your contribution and congratulations on your first contribution to the Symfony documentation 🎉 I made some minor adjustements while merging your PR 👍

[Nyholm]

Wohoo!