Resolve useless_conversion clippy lint in test

    error: explicit call to `.into_iter()` in function argument accepting `IntoIterator`
       --> test_suite/tests/test_de.rs:202:12
        |
    202 |     .chain(vec![Token::MapEnd].into_iter())
        |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider removing the `.into_iter()`: `vec![Token::MapEnd]`
        |
    note: this parameter accepts any `IntoIterator`, so you don't need to call `.into_iter()`
       --> /home/david/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/iter/traits/iterator.rs:522:12
        |
    522 |         U: IntoIterator<Item = Self::Item>,
        |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#useless_conversion
        = note: `-D clippy::useless-conversion` implied by `-D clippy::all`

Author: dtolnay

test_suite/tests/test_de.rs

@@ -199,7 +199,7 @@ fn assert_de_tokens_ignore(ignorable_tokens: &[Token]) {
     ]
     .into_iter()
     .chain(ignorable_tokens.iter().copied())
-    .chain(vec![Token::MapEnd].into_iter())
+    .chain(vec![Token::MapEnd])
     .collect();
 
     let expected = IgnoreBase { a: 1 };