tests: Fix dead_code warning for tuple struct

```
error: field `0` is never read
   --> tests/stream.rs:264:14
    |
264 | pub struct A(i32);
    |            - ^^^
    |            |
    |            field in this struct
    |
    = note: `-D dead-code` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(dead_code)]`
help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field
    |
264 | pub struct A(());
    |              ~~
```

tests/stream.rs

@@ -261,7 +261,7 @@ pub trait Trait {
     async fn stream4(&self);
 }
 
-pub struct A(i32);
+struct A;
 
 impl Trait for A {
     #[stream(boxed, item = i32)]
@@ -325,7 +325,7 @@ fn test_early_exit() {
         }
     }
 
-    #[stream(item=i32)]
+    #[stream(item = i32)]
     async fn early_exit_block() {
         let s = {
             #[stream]