Make tests compatible with production Docker environment

[vyahhi]

We run Hyperstyle in Docker (based on https://github.com/hyperskill/hyperstyle/blob/main/Dockerfile) on Stepik. Currently, tests do not pass within the production environment since you run tests in a different environment for GitHub Actions (https://github.com/hyperskill/hyperstyle/blob/main/.github/workflows/build.yml).

Please do the following:

1. Use the same Docker environment to run tests.
2. Fix all failing test cases (some for eslint and flake8) in main and main-upd branches.