let_chains: Adjust tests for pre-expansion gating.

Author: Centril

src/test/ui/rfc-2497-if-let-chains/disallowed-positions.rs

@@ -17,7 +17,10 @@
 //
 // To that end, we check some positions which is not part of the language above.
 
-#![feature(const_generics)] //~ WARN the feature `const_generics` is incomplete
+#![feature(const_generics)]
+//~^ WARN the feature `const_generics` is incomplete
+#![feature(let_chains)] // Avoid inflating `.stderr` with overzealous gates in this test.
+//~^ WARN the feature `let_chains` is incomplete
 
 #![allow(irrefutable_let_patterns)]