Use `mockito-scala` instead of just `mockito` for mocks

[sergey-pudov]

Description

Currently we are using pure Java implementation of mockito mocking library, which lacks some nice and useful features. Seems like Scala implementation wasn't available at the moment of development.

We should start to use mockito-scala instead.

Link to the library: https://github.com/mockito/mockito-scala

Acceptance criteria:

• mockito-scala installed as dependency
• Few tests are re-written using new library
• Agreement to use it for future tests is established
• Refactoring tickets for tests are created into backlog