[IMP] Install testing requirements into separate image.

[amh-mw]

Fixes #277

[amh-mw]

I'm still playing around with this a bit, so I've included two versions of the work.

In version one, the main Dockerfile is converted into a multi-stage build, which would require existing build processes to add --target=web.

In version two, an entirely separate test.Dockerfile is created. This is more similar to what my own build process currently looks like.

[amh-mw]

Some workflows to consider:

When I'm developing locally, I don't want to repeatedly reinstall chromium, so my image inheritance looks like:
odoo:15 -> odoo:test -> me:latest

When I'm pushing code, I don't want chromium in production, but I want to test an image that is very similar:
odoo:15 -> me:latest -> me:test
Run tests on me:test, but push me:latest into the image repository.

[amh-mw]

This is working nicely for me with a .travis.yml containing:

script:
  - docker build . --tag=me:travis || travis_terminate 1
  - docker build . -f test.Dockerfile --build-arg=image=me:travis --tag=me:test || travis_terminate 1
  - docker-compose -f docker-compose.travis.yaml run test || travis_terminate 1