fix a clippy test

Author: RalfJung

src/tools/clippy/tests/ui/indexing_slicing_index.rs

@@ -3,7 +3,7 @@
 // We also check the out_of_bounds_indexing lint here, because it lints similar things and
 // we want to avoid false positives.
 #![warn(clippy::out_of_bounds_indexing)]
-#![allow(const_err, clippy::no_effect, clippy::unnecessary_operation)]
+#![allow(const_err, unconditional_panic, clippy::no_effect, clippy::unnecessary_operation)]
 
 const ARR: [i32; 2] = [1, 2];
 const REF: &i32 = &ARR[idx()]; // Ok, should not produce stderr.