Add Status UNKOWN for skipped tests with 'unknown' reason

[aleksandr-kotlyar]

What?

Add Status UNKOWN for skipped tests with 'unknown' reason

Why?

To colorize tests which are not triaged yet in category unknown in allure history trends.

Usage example

Tests skipped with 'unknown' reason will be colored purple in allure history trends.

@pytest.mark.skip('unknown')
def test_needs_to_be_triaged():
    """Or test which is not completed yet."""

Checklist

• [x] Sign Allure CLA
• [ ] Provide unit tests

[aleksandr-kotlyar]

If it will pass review - please, squash commits on merge :)