Add GPIB::3 and GPIB::4 resources to Keithley 2600 sim file

docs/changes/newsfragments/8756.improved_driver

@@ -0,0 +1,2 @@
+The `Keithley_2600.yaml` pyvisa-sim file now also defines the resources `GPIB::3::INSTR` and `GPIB::4::INSTR`,
+so that up to four simulated Keithley 2600 instruments with independent state can be used at the same time.

src/qcodes/instrument/sims/Keithley_2600.yaml

@@ -203,3 +203,9 @@ resources:
 
   GPIB::2::INSTR:
     device: Keithley
+
+  GPIB::3::INSTR:
+    device: Keithley
+
+  GPIB::4::INSTR:
+    device: Keithley

tests/drivers/test_keithley_26xx.py

@@ -8,6 +8,7 @@
 
 from qcodes.dataset import LinSweep
 from qcodes.instrument_drivers.Keithley import (
+    Keithley2600,
     Keithley2600MeasurementStatus,
     Keithley2614B,
 )
@@ -47,6 +48,29 @@ def test_idn(driver: Keithley2614B) -> None:
     } == driver.IDN()
 
 
+def test_four_sim_addresses_have_independent_state() -> None:
+    voltages = {1: 1.5, 2: -0.5, 3: 0.25, 4: 2.0}
+    drivers: list[Keithley2600] = []
+    try:
+        for gpib_address in voltages:
+            drivers.append(
+                Keithley2600(
+                    f"Keithley_2600_{gpib_address}",
+                    address=f"GPIB::{gpib_address}::INSTR",
+                    pyvisa_sim_file="Keithley_2600.yaml",
+                )
+            )
+
+        for driver, voltage in zip(drivers, voltages.values(), strict=True):
+            driver.smua.volt(voltage)
+
+        for driver, voltage in zip(drivers, voltages.values(), strict=True):
+            assert driver.smua.volt() == voltage
+    finally:
+        for driver in drivers:
+            driver.close()
+
+
 def test_smu_channels_and_their_parameters(driver: Keithley2614B) -> None:
     assert {"smua", "smub"} == set(driver.submodules.keys())