Move the unit-tests for flagging PR to their own file and expand them
    
    This commit in addition to moving the old tests to their own file also
    restructures them so they do less and are better documented.
    It also add some tests for the notifications being sent when a project
    opts in to notification on PR flags.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>