'return a TAP message string.
:param success: True for positive message string.
:param counter: number of TAP line in plan.
:param message: additional message to report in TAP line.'

@classmethod
def tap_ok(self, success, counter, message):

    if success:
        message = ('ok    %s    -    %s' % (counter, message))
    else:
        message = ('not    ok    %s    -    %s' % (counter, message))
    return message
