Ignore clippy::test_attr_in_doctest lint

```
error: unit tests in doctest are not executed
  --> futures-test/src/lib.rs:77:5
   |
77 |   /// #[test]
   |  _____^
78 | | /// fn my_test() {
   | |______________^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#test_attr_in_doctest
   = note: `-D clippy::test-attr-in-doctest` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(clippy::test_attr_in_doctest)]`
```

Author: taiki-e

futures-test/src/lib.rs

@@ -14,6 +14,7 @@
         allow(dead_code, unused_assignments, unused_variables)
     )
 ))]
+#![allow(clippy::test_attr_in_doctest)]
 
 #[cfg(not(feature = "std"))]
 compile_error!(