make sure compile-fail tests would compile if we screw up

Author: RalfJung

tests/compile-fail-fullmir/stacked_borrows/box_exclusive_violation1.rs

@@ -25,5 +25,5 @@ fn unknown_code_2() { unsafe {
 } }
 
 fn main() {
-    assert_eq!(demo_mut_advanced_unique(Box::new(0)), 5);
+    demo_mut_advanced_unique(Box::new(0));
 }

tests/compile-fail-fullmir/stacked_borrows/mut_exclusive_violation1.rs

@@ -25,5 +25,5 @@ fn unknown_code_2() { unsafe {
 } }
 
 fn main() {
-    assert_eq!(demo_mut_advanced_unique(&mut 0), 5);
+    demo_mut_advanced_unique(&mut 0);
 }