proxy: add tests for batch http sql (#4793)

This PR adds an integration test case for batch HTTP SQL endpoint.
https://github.com/neondatabase/neon/pull/4654/ should be merged first
before we land this PR.

---------

Signed-off-by: Alex Chi Z <chi@neon.tech>

proxy/src/http/sql_over_http.rs

@@ -223,7 +223,7 @@ pub async fn handle(
                 }
             }
             transaction.commit().await?;
-            Ok(Value::Array(results))
+            Ok(json!({ "results": results }))
         }
     };