clarify test purpose

tests/pass/stacked-borrows/zst-field-retagging-terminates.rs

@@ -1,4 +1,5 @@
 //@compile-flags: -Zmiri-retag-fields
+// Checks that the test does not run forever (which relies on a fast path).
 fn main() {
     let array = [(); usize::MAX];
     drop(array); // Pass the array to a function, retagging its fields