Add test of flatten in enum

    error: collection is never read
       --> test_suite/tests/test_gen.rs:728:25
        |
    728 |     #[derive(Serialize, Deserialize)]
        |                         ^^^^^^^^^^^
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#collection_is_never_read
        = note: this error originates in the derive macro `Deserialize` (in Nightly builds, run with -Z macro-backtrace for more info)

test_suite/tests/test_gen.rs

@@ -725,6 +725,19 @@ fn test_gen() {
         flat: T,
     }
 
+    #[derive(Serialize, Deserialize)]
+    #[serde(untagged)]
+    pub enum Inner<T> {
+        Builder {
+            s: T,
+            #[serde(flatten)]
+            o: T,
+        },
+        Default {
+            s: T,
+        },
+    }
+
     // https://github.com/serde-rs/serde/issues/1804
     #[derive(Serialize, Deserialize)]
     pub enum Message {