Try to remove the old mock logs first.

def start(self):
        """
        Try to remove the old mock logs first.
        """
        if self.resultdir:
            for lname in self.mock_logfiles:
                self.logfiles[lname] = self.build.path_module.join(self.resultdir,
                                                                   lname)
        else:
            for lname in self.mock_logfiles:
                self.logfiles[lname] = lname
        self.addLogObserver('state.log', MockStateObserver())

        cmd = remotecommand.RemoteCommand('rmdir', {'dir':
                                                    [self.build.path_module.join('build', self.logfiles[l])
                                                     for l in self.mock_logfiles]})
        d = self.runCommand(cmd)
        # must resolve super() outside of the callback context.
        super_ = super()

        @d.addCallback
        def removeDone(cmd):
            super_.start()
        d.addErrback(self.failed)