Unit tests refactoring for all modules

[DenisRybas]

Scope:

• Unit tests from all modules
• Two goals:
  • check test coverage
  • refactoring/simplification

Details:

• make sure Unit Tests coverage is enough
• simplify the unit tests (simplify initialisation):

  • simplify initialisation and test stricture by creating helper functions, for example

    ```
    // prepare
    add_vendor()
    add_root_cert(cert_path)
    
    // execution
    add_revocation_point()
    
    //check
    asserts... 
    ```
    

  • get rid of magic numbers (for example, make sure VID constant is issued instead of hard-coded values)

• improve error messages (all error text needs to be in errors.go's NewXXX methods, not error strings created on the fly).