Cleanup ReadWriteMultipleRegistersResponse and testing

[alexrudd2]

There is no reason ReadWriteMultipleRegistersResponse can't inherit from ReadHoldingRegistersResponse. The sole differences are:
(1) the function code (OK)
(2) decode() not initializing self.registers

pymodbus/pymodbus/pdu/register_read_message.py

Lines 385 to 392 in d329475

	def decode(self, data):
	"""Decode the register response packet.
	:param data: The response to decode
	"""
	bytecount = int(data[0])
	for i in range(1, bytecount, 2):
	self.registers.append(struct.unpack(">H", data[i : i + 2])[0])

This is a bug - if for some dumb reason decode() is called twice, it will keep appending registers to self.registers. As far as I can tell, this bug (and corresponding incorrect test code) has been there since b02f5fe (13 years!)

Changing this also makes it apparent that the test code can be readily simplified.

[janiversen]

Wow, nice cleanup!