Add test for 2021 prelude.

Author: m-ou-se

src/test/ui/prelude2021.rs

@@ -0,0 +1,7 @@
+// check-pass
+// edition:2021
+// compile-flags: -Zunstable-options
+
+fn main() {
+    let _: u16 = 123i32.try_into().unwrap();
+}