Testing process is not documented and possibly broken

[Vlad-Shcherbina]

I was trying to make a minor change but couldn't get the tests running. There are no testing instructions in the README or similar places.

The CI config suggests cargo test --all-features, but this didn't work locally for me. Also noticed the CI builds are consistently failing.

Could you please document how to test this project locally during development? Adding this to the README would make it easier for potential contributors.