Add processing for additional tests status 'skipped'

In models for enum attribute of Test entity new value has been added.
addError hook method of StoragePlugin has been modified with purpose to
process new result state of executed tests which now can be 'skipped'.
Unit and functional tests were updated.

Change-Id: I593d67eed12a9a20ba795501a1f8cceedfbb4587
Implements: blueprint additional-tests-statuses