test(mavsdk): leave tests backwards compatible but enable SIH

This reverts some of the changes to keep runners of classic Gazebo in
there but leave it optional, so we can still use it if required.

test/mavsdk_tests/integration_test_runner/process_helper.py

@@ -168,7 +168,7 @@ class Px4Runner(Runner):
                 os.path.join(workspace_dir, "test_data"),
                 "-d"
             ]
-        self.env["PX4_SIM_MODEL"] = "sihsim_quadx"
+        self.env["PX4_SIM_MODEL"] = model
         self.env["PX4_SIM_SPEED_FACTOR"] = str(speed_factor)
         self.debugger = debugger
         self.clear_rootfs()