Task dev dependencies #356

I recently tried to use my go-to pytest-mock in a test for tasks and realized I couldn't add dev dependencies to a task. I then realized that pytest is being installed inside all task containers, which I couldn't understand for the life of me until I saw it was being smuggled in with the metr-task-standard. I'd like to propose the following:

1. We add another task-test stage to the dockerfile that extends the final task stage. This stage installs pytest and also looks for requirements.dev.txt and installs them.
2. Use the task-test stage when building the docker image for viv task test
3. Remove pytest as a dependency of metr-task-standard