tests/test_modbusrtudriver: mock serial instead of relying on port=None

Since minimalmodbus 2.1.0, minimalmodbus.Instrument() [1] no longer
accepts port=None. Accepting this was never documented. The idea was
that port=None would be passed to serial.Serial(port=None), which
explicitly allows this [2].

Now that this is no longer allowed, stick to the original
port="/dev/tty0" in the test and mock serial.Serial, so it is not
actually opened.

[1] https://minimalmodbus.readthedocs.io/en/stable/apiminimalmodbus.html#minimalmodbus.Instrument
[2] https://pyserial.readthedocs.io/en/latest/pyserial_api.html#serial.Serial

Signed-off-by: Bastian Krause <bst@pengutronix.de>

tests/test_modbusrtudriver.py

@@ -24,12 +24,11 @@ def test_resource_with_non_default_argument(target):
 
 def test_driver(target, mocker):
     pytest.importorskip("minimalmodbus")
+    mocker.patch('serial.Serial')
+
     ModbusRTU(target, name=None, port="/dev/tty0", address=10)
     driver = ModbusRTUDriver(target, name=None)
 
-    # Ensure pyserial will not try to open the port
-    driver.resource.port = None
-
     target.activate(driver)
 
     assert driver.instrument.serial.baudrate == 115200