Use a different kotlintest spec for tests

[serpro69]

Should consider using a different spec other than FreeSpec for unit tests.

Right now with the FreeSpec the tests are organized as:

"GIVEN xxx" - {
    // some preconditions

    "WHEN yyy" - {
        // some action

        "THEN zzz" { 
            // verification of results
        }
    }
}

However there is very little way to enforce this structure.
It would be better to use some specs where you need to use a pre-defined functions (i.e. FeatureSpec, BehaviorSpec or DescribeSpec) instead of strings.