tests/util/test_app: Increase default async_pool_size

With the background worker ported to `deadpool` it requires one extra connection, so setting it to two connections by default isn't sufficient anymore.

src/tests/util/test_app.rs

@@ -396,7 +396,7 @@ fn simple_config() -> config::Server {
             url: String::from("invalid default url").into(),
             read_only_mode: false,
             pool_size: 3,
-            async_pool_size: 2,
+            async_pool_size: 3,
             min_idle: None,
         },
         replica: None,