Provide a way to waive the absence of a test result
    
    This change is needed to submit a new waiver
    (using the API) for tests which do not have a result in ResultDB.
    To achieve this we replace the result_id with a ResultDB subject and a
    ResultDB testcase
    
    The changes include:
    * api changes
    * database migration of older waivers
    * changes to tests so they worked with new api