Fix clippy::useless_conversion warning

```
warning: explicit call to `.into_iter()` in function argument accepting `IntoIterator`
  --> futures/tests/stream_select_all.rs:83:38
   |
83 | ...ec![stream::iter(vec![1].into_iter()), stream::iter(vec![2].i...
   |                     ^^^^^^^^^^^^^^^^^^^ help: consider removing the `.into_iter()`: `vec![1]`
   |
note: this parameter accepts any `IntoIterator`, so you don't need to call `.into_iter()`
  --> /Users/taiki/projects/sources/rust-lang/futures-rs/futures-util/src/stream/iter.rs:31:8
   |
31 |     I: IntoIterator,
   |        ^^^^^^^^^^^^
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#useless_conversion
   = note: `#[warn(clippy::useless_conversion)]` on by default

warning: explicit call to `.into_iter()` in function argument accepting `IntoIterator`
  --> futures/tests/stream_select_all.rs:83:73
   |
83 | ...)), stream::iter(vec![2].into_iter())]);
   |                     ^^^^^^^^^^^^^^^^^^^ help: consider removing the `.into_iter()`: `vec![2]`
   |
note: this parameter accepts any `IntoIterator`, so you don't need to call `.into_iter()`
  --> /Users/taiki/projects/sources/rust-lang/futures-rs/futures-util/src/stream/iter.rs:31:8
   |
31 |     I: IntoIterator,
   |        ^^^^^^^^^^^^
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#useless_conversion

warning: explicit call to `.into_iter()` in function argument accepting `IntoIterator`
  --> futures/tests/stream_select_all.rs:91:29
   |
91 |     tasks.push(stream::iter(vec![3].into_iter()));
   |                             ^^^^^^^^^^^^^^^^^^^ help: consider removing the `.into_iter()`: `vec![3]`
   |
note: this parameter accepts any `IntoIterator`, so you don't need to call `.into_iter()`
  --> /Users/taiki/projects/sources/rust-lang/futures-rs/futures-util/src/stream/iter.rs:31:8
   |
31 |     I: IntoIterator,
   |        ^^^^^^^^^^^^
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#useless_conversion

warning: explicit call to `.into_iter()` in function argument accepting `IntoIterator`
   --> futures/tests/async_await_macros.rs:325:41
    |
325 | ...ct!(stream::iter(vec![1].into_iter()), stream::iter(vec![1]....
    |                     ^^^^^^^^^^^^^^^^^^^ help: consider removing the `.into_iter()`: `vec![1]`
    |
note: this parameter accepts any `IntoIterator`, so you don't need to call `.into_iter()`
   --> /Users/taiki/projects/sources/rust-lang/futures-rs/futures-util/src/stream/iter.rs:31:8
    |
31  |     I: IntoIterator,
    |        ^^^^^^^^^^^^
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#useless_conversion
    = note: `#[warn(clippy::useless_conversion)]` on by default

warning: explicit call to `.into_iter()` in function argument accepting `IntoIterator`
   --> futures/tests/async_await_macros.rs:325:76
    |
325 | ...)), stream::iter(vec![1].into_iter()));
    |                     ^^^^^^^^^^^^^^^^^^^ help: consider removing the `.into_iter()`: `vec![1]`
    |
note: this parameter accepts any `IntoIterator`, so you don't need to call `.into_iter()`
   --> /Users/taiki/projects/sources/rust-lang/futures-rs/futures-util/src/stream/iter.rs:31:8
    |
31  |     I: IntoIterator,
    |        ^^^^^^^^^^^^
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#useless_conversion
```

Author: taiki-e

futures/tests/async_await_macros.rs

@@ -321,8 +321,7 @@ fn stream_select() {
         assert_eq!(endless_ones.next().await, Some(1));
         assert_eq!(endless_ones.next().await, Some(1));
 
-        let mut finite_list =
-            stream_select!(stream::iter(vec![1].into_iter()), stream::iter(vec![1].into_iter()));
+        let mut finite_list = stream_select!(stream::iter(vec![1]), stream::iter(vec![1]));
         assert_eq!(finite_list.next().await, Some(1));
         assert_eq!(finite_list.next().await, Some(1));
         assert_eq!(finite_list.next().await, None);

futures/tests/stream_select_all.rs

@@ -79,16 +79,15 @@ fn works_1() {
 
 #[test]
 fn clear() {
-    let mut tasks =
-        select_all(vec![stream::iter(vec![1].into_iter()), stream::iter(vec![2].into_iter())]);
+    let mut tasks = select_all(vec![stream::iter(vec![1]), stream::iter(vec![2])]);
 
     assert_eq!(block_on(tasks.next()), Some(1));
     assert!(!tasks.is_empty());
 
     tasks.clear();
     assert!(tasks.is_empty());
 
-    tasks.push(stream::iter(vec![3].into_iter()));
+    tasks.push(stream::iter(vec![3]));
     assert!(!tasks.is_empty());
 
     tasks.clear();