Further simplify start tracking error setup

xfail directly in the probe rather than awkwardly relying on truthy response from the method

android/tests_backend/hardware/location.py

@@ -106,5 +106,4 @@ def setup_location_error(self):
         pass
 
     def setup_tracking_start_error(self):
-        # errors do not occur when tracking starts on Android
-        pass
+        pytest.xfail("Tracking start cannot fail on Android")

cocoa/tests_backend/hardware/location.py

@@ -1,5 +1,6 @@
 from unittest.mock import Mock, PropertyMock
 
+import pytest
 from rubicon.objc import ObjCClass
 
 from toga_cocoa import libs as cocoa
@@ -179,5 +180,4 @@ def setup_location_error(self):
         pass
 
     def setup_tracking_start_error(self):
-        # errors do not occur when tracking starts on macOS
-        pass
+        pytest.xfail("Tracking start cannot fail on macOS")

gtk/tests_backend/hardware/location.py

@@ -165,8 +165,6 @@ def setup_location_error(self):
 
     def setup_tracking_start_error(self):
         self.mock_native.client.props.active = False
-        # Return True to indicate tracking start errors are setup
-        return True
 
     async def simulate_location_error(self, location):
         # No simulation required after setup_location_error, wait for the location
@@ -201,8 +199,6 @@ def _xfail_location_portal_no_post_init_failure(self):
 
     def setup_tracking_start_error(self):
         self._xfail_location_portal_no_post_init_failure()
-        # Return True to indicate tracking start errors are setup
-        return True
 
     def setup_location_error(self):
         self._xfail_location_portal_no_post_init_failure()

iOS/tests_backend/hardware/location.py

@@ -1,5 +1,6 @@
 from unittest.mock import Mock, PropertyMock
 
+import pytest
 from rubicon.objc import ObjCClass
 
 from toga_iOS import libs as iOS
@@ -179,5 +180,4 @@ def setup_location_error(self):
         pass
 
     def setup_tracking_start_error(self):
-        # errors do not occur when tracking starts on iOS
-        pass
+        pytest.xfail("Tracking start cannot fail on iOS")

testbed/tests/hardware/test_location.py

@@ -288,11 +288,6 @@ async def test_location_error(app, location_probe):
 
 async def test_location_tracking_start_error(app, location_probe):
     """If location tracking fails to start, location raises an error."""
-    if not location_probe.setup_tracking_start_error():
-        return pytest.xfail(
-            f"Tracking start cannot fail on {toga.platform.current_platform}"
-        )
-
     # Ensure location has permissions
     location_probe.grant_permission()