Verify that peeps stay on paths

As per PR recommendation, confirm that a peep is still on the footpath for every step of the pathfinding tests.

test/tests/Pathfinding.cpp

@@ -117,6 +117,10 @@ protected:
             pos->z = peep->z / 8;
 
             EXPECT_PRED_FORMAT1(AssertIsNotForbiddenPosition, *pos);
+
+            // Check that the peep is still on a footpath. Use next_z instead of pos->z here because pos->z will change
+            // when the peep is halfway up a slope, but next_z will not change until they move to the next tile.
+            EXPECT_NE(map_get_footpath_element(pos->x, pos->y, peep->next_z), nullptr);
         }
 
         // Clean up the peep, because we're reusing this loaded context for all tests.