add mysql dialect tag for query failing in Doltgres

Author: angelamayxie

enginetest/queries/script_queries.go

@@ -11114,7 +11114,10 @@ where
 	},
 
 	{
-		Name: "correct union field index",
+		// TODO: This test currently fails in Doltgres because Doltgres does not allow `create table...as select...`
+		// even though it's a valid Postgres query. Remove Dialect tag once fixed in Doltgres
+		Dialect: "mysql",
+		Name:    "union field indexes",
 		SetUpScript: []string{
 			"create table t(id int primary key auto_increment, words varchar(100))",
 			"insert into t(words) values ('foo'),('bar'),('baz'),('zap')",