Add get_line confusable to Stdin::read_line()

Add tests for addition of `#[rustc_confusables("get_line")]`

std/src/io/stdio.rs

@@ -394,6 +394,7 @@ impl Stdin {
     ///   in which case it will wait for the Enter key to be pressed before
     ///   continuing
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_confusables("get_line")]
     pub fn read_line(&self, buf: &mut String) -> io::Result<usize> {
         self.lock().read_line(buf)
     }