Update FlutterEngineTest.mm

Author: hannah-hyj

shell/platform/darwin/macos/framework/Source/FlutterEngineTest.mm

@@ -774,6 +774,10 @@ - (nonnull NSView*)createWithViewIdentifier:(int64_t)viewId arguments:(nullable
                                       withPriority:NSAccessibilityPriorityMedium])
       .andDo((^(NSInvocation* invocation) {
         announced = TRUE;
+        [invocation retainArguments];
+        NSString* message;
+        [invocation getArgument:&message atIndex:2];
+        EXPECT_EQ(message, @"error message");
       }));
 
   NSDictionary<NSString*, id>* annotatedEvent =