Add support for standard test interface
    
    The changes in this commit are:
      - change the file we discover from 'runtask.yml' to 'tests.yml'
      - check for existance of a repo before attempting to clone it
      - removal of checks and tests for the scheduling part of old forumulae
    
    Fixes https://pagure.io/taskotron/taskotron-trigger/issue/39