Improve local testing

[JakNowy]

I wanted to pass a specific test to be run with pytest. Initially I came with modifying test.sh script so it accepts additional parameter. Then noticed it's already supported in tests-start.sh.

This PR therefore updates the backend README.md, advising you to run more flexible tests-start.sh which calls the other under the hood. Now that a single test can be run (useful in development), I'm also enabling --no-coverage flag for skipping coverage calculation during development. Additional --message argument is being introduced to ensure alignment with CI coverage message.