[backend] LogHandler: don't drop exc_info from LogRecord
    
    Python's internals depend on that even before the message is
    emitted, exception like this is raised:
    
        Traceback (most recent call last):
          File "/usr/lib64/python3.7/logging/__init__.py", line 1034, in emit
            msg = self.format(record)
          File "/usr/lib64/python3.7/logging/__init__.py", line 880, in format
            return fmt.format(record)
          File "/usr/lib64/python3.7/logging/__init__.py", line 623, in format
            if record.exc_info:
        AttributeError: 'LogRecord' object has no attribute 'exc_info'
    
    Update test_helpers.py so we have also a small test for the
    delivered message format.