[Testing] Minor tweaks in the main testing article

[javiereguiluz]

@wouterj do you know why we use .. code-block:: env instead of .. code-block:: bash for the highlighting of .env files? I don't know if env highlighting is standard or if we had to do something to support it ... but bash seems more natural and it should be standard. Thanks.

[wouterj]

env behaves just like php-symfony and all other highlighting aliases used in the Symfony docs. The highlighter is run with language bash, but for the renderer it's env. This means we can customize the label in the configuration blocks (how we use php-symfony/php-standalone), but also have different styling of the blocks.

That latter is important for env/bash: we highlight bash code blocks with a black background in the light theme (to fit the terminal frame), but we highlight env code blocks just like any other code block.

[javiereguiluz]

Thanks for the explanation. About highlighting, please note that we changed this recently and bash blocks no longer look like terminal blocks.