Add tests for Stepper component

[obulat]

Signed-off-by: Olga Bulat obulat@gmail.com

Fixes

Fixes #126 by @akmadian

Description

Unit and e2e tests for Stepper component.

Technical details

Unit tests have 100% coverage of the Stepper component.
The e2e tests use page objects to make tests more readable.

This PR adds two changes to the code: removes obsolete handling of the step-header click for steps that the user hasn't interacted with yet, and adds stepName to step-header classes to make testing easier.

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the master branch of the repository.
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.

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