Remove `skip` from the `ResultInterpret` enumeration

[psss]

As discussed today, the skip value was never documented in the test result key specification and was added by an accident.

There seems to be a use case to support skipping selected tests but that functionality is substantially different from the result interpretation configured by the result key which happens only after test execution is completed so most probably a new test key should be introduced for it.

Pull Request Checklist

• [x] implement the feature
• [x] modify the json schema