Run process-distgit in the build root if necessary
    
    Steals the `broot` object from the BuildSRPMFromSCMTask.handler() caller
    if it is not supplied to the callback as the parameter `buildroot`.
    
    Adapt tests accordingly, mainly: because the bulk of the work is done
    using the CLI in the build root, move testing it into the tests for
    `rpmautospec.process_distgit`.
    
    Signed-off-by: Nils Philippsen <nils@redhat.com>