Resolve new dead_code warnings in test suite

Bisects to rust-lang/rust#141407.

    warning: struct `Struct` is never constructed
       --> test_suite/tests/test_gen.rs:803:16
        |
    803 |     pub struct Struct {
        |                ^^^^^^
        |
        = note: `#[warn(dead_code)]` on by default

    warning: function `vec_first_element` is never used
       --> test_suite/tests/test_gen.rs:885:4
        |
    885 | fn vec_first_element<T, S>(vec: &[T], serializer: S) -> StdResult<S::Ok, S::Error>
        |    ^^^^^^^^^^^^^^^^^

    warning: struct `S` is never constructed
     --> test_suite/tests/regression/issue2415.rs:5:12
      |
    5 | pub struct S;
      |            ^
      |
      = note: `#[warn(dead_code)]` on by default

Author: dtolnay

test_suite/tests/regression/issue2415.rs

@@ -2,4 +2,5 @@ use serde_derive::Serialize;
 
 #[derive(Serialize)]
 #[serde()]
+#[allow(dead_code)]
 pub struct S;

test_suite/tests/test_gen.rs

@@ -805,6 +805,8 @@ fn test_gen() {
         pub vec: Vec<Self>,
     }
 
+    assert_ser::<Struct>();
+
     #[derive(Deserialize)]
     #[serde(bound(deserialize = "[&'de str; N]: Copy"))]
     pub struct GenericUnitStruct<const N: usize>;