postgres: be explicit about an expression type for a test

sqlx-test/src/lib.rs

@@ -119,6 +119,6 @@ macro_rules! Sqlite_query_for_test_prepared_type {
 #[macro_export]
 macro_rules! Postgres_query_for_test_prepared_type {
     () => {
-        "SELECT {0} is not distinct from $1, $2::text as _1, {0}, $3 as _3"
+        "SELECT {0} is not distinct from $1, $2::text as _1, {0} as _2, $3 as _3"
     };
 }