S allius/issue334 (#335)

* move forward_at_cmd_resp into InfosG3P class

- the variable is shared between the two connections
of an inverter. One is for the TSUN cloud and the
other for the device.

* use inverter class to share values between
the two protocol instances of a proxy
- move forward_at_cmd_resp into class InverterG3P
- store inverter ptr in Solarman_V5 instances
- add inverter ptr to all constructurs of protocols
- adapt doku and unit tests-
- add integration tests for AT+ commands which
  check the forwarding from and to the TSUN cloud

* adapt and improve the unit tests
- fix node_id declaration, which always has a / at
  the end. See config grammar for this rule
- set global var test to default after test run

app/tests/test_modbus_tcp.py

@@ -190,7 +190,8 @@ def patch_mqtt_except():
         yield conn
 
 @pytest.mark.asyncio
-async def test_modbus_conn(patch_open):
+async def test_modbus_conn(config_conn, patch_open):
+    _ = config_conn
     _ = patch_open
     assert Infos.stat['proxy']['Inverter_Cnt'] == 0
 
@@ -210,6 +211,7 @@ async def test_modbus_conn(patch_open):
 
 @pytest.mark.asyncio
 async def test_modbus_no_cnf():
+    _ = config_conn
     assert Infos.stat['proxy']['Inverter_Cnt'] == 0
     loop = asyncio.get_event_loop()
     ModbusTcp(loop)