Improve type-security with phpstan and set up phpunit

[alsciende]

The goal is to prepare an upgrade of Symfony from the 3.x version to an up-to-date 8.x version. For this, we need a better test suite to prevent regression. So phpstan and phpunit need to strongly constraint future code evolutions.

What this PR changes in the code:

• use phpstan level 3 to improve types and fix a few bugs in older controllers
• convert deprecated YAML mappings to XML mappings for Doctrine 3.x
• set up phpunit with a test container and a test database
• remove doctrine/doctrine-fixtures-bundle which is not used anymore
• include a mysql dump for the test database

What this PR brings to the table:

• a test container named nrdb-test which is distinct from the dev container, and is used to run phpunit (and other test tools that would need Symfony in a test env)
• a test database named nrdb-test-db which is distinct from the test database, and is automatically loaded with some testing fixtures commited to tests/fixtures/mysqldump. The tests use these fixtures to be reproducible and consistant.

To run the tests, execute this from your nrdb-dev-env directory:

docker compose exec -it -u www-data nrdb-test bash -c 'make test'