Simplify setup of local tests & update contributor docs

[tholor]

Is your feature request related to a problem? Please describe.
Our CI works nicely, but it's currently quite cumbersome to run all tests locally. Especially, as they require running some document stores in the background.

Describe the solution you'd like

• Cleaner option to launch required document stores locally
• Remove unused fixtures
• Review if we can make better use of the --document_store param to run all tests with the docstore of ones choice (e.g. document_store=memory should cover all major functionalities and only leave out the tests that are highly specific for elastic, faiss & co)
• Update contributor guidelines to explain the local test setup

[mathislucka]

Just to add a bit of information to this discussion:

I think what's still missing from the documentation added in #1355 is a hint that a dev should first install the requirements.txt and the requirements-dev.txt (pytest is only included in the dev requirements).

Furthermore, your Dockerfile does not install the requirements-dev.txt which would mean that the container would at least be missing pytest and sentence-transformers.

Also if using the docker container shouldn't the tests be run inside the container? Otherwise I'd still have to install all the dependencies locally. So the docs should probably contain a section on SSH'ing into the container and running tests from a shell inside the container.

Hope I'm not completely off with my observations.

[tholor]

Fixed by #1487