Ignore dead code lint in tests

New in nightly-2024-03-24 from rust-lang/rust#119552.

    warning: field `x` is never read
      --> tests/regression/issue795.rs:11:15
       |
    11 |     Variant { x: u8 },
       |     -------   ^
       |     |
       |     field in this variant
       |
       = note: `#[warn(dead_code)]` on by default

    warning: field `i` is never read
      --> tests/regression/issue845.rs:63:9
       |
    61 | pub struct Struct {
       |            ------ field in this struct
    62 |     #[serde(deserialize_with = "deserialize_integer_or_string")]
    63 |     pub i: i64,
       |         ^
       |
       = note: `Struct` has a derived impl for the trait `Debug`, but this is intentionally ignored during dead code analysis

tests/regression/issue795.rs

@@ -8,7 +8,10 @@ use std::fmt;
 
 #[derive(Debug)]
 pub enum Enum {
-    Variant { x: u8 },
+    Variant {
+        #[allow(dead_code)]
+        x: u8,
+    },
 }
 
 impl<'de> Deserialize<'de> for Enum {

tests/regression/issue845.rs

@@ -60,6 +60,7 @@ where
 #[derive(Deserialize, Debug)]
 pub struct Struct {
     #[serde(deserialize_with = "deserialize_integer_or_string")]
+    #[allow(dead_code)]
     pub i: i64,
 }