Mark deduplicated errors as expected in gate test

Author: LeSeulArtichaut

src/test/ui/feature-gates/feature-gate-unsafe_block_in_unsafe_fn.rs

@@ -1,4 +1,6 @@
 #![deny(unsafe_op_in_unsafe_fn)]
 //~^ ERROR the `unsafe_op_in_unsafe_fn` lint is unstable
+//~| ERROR the `unsafe_op_in_unsafe_fn` lint is unstable
+//~| ERROR the `unsafe_op_in_unsafe_fn` lint is unstable
 
 fn main() {}