test: stop using pytest-dotenv

[peterschutt]

Thinking we should stop using pytest-dotenv altogether and load all env_files specified in config ourselves in the plugin. That should take the plugin ordering out of the equation altogether.

Keep the same API for specifying dotenv files in pyproject as pytest-dotenv.

Only load files specified in pyproject.toml or default, without overwriting existing.

We can have instructions on how to use the plugin alongside pytest-env in the docs.