Create ZST-nthback.rs test for nth_back method

Add test for Intolter::nth_back with ZSTs

Author: DrAsu33

tests/ui/iterators/ZST-nthback.rs

@@ -0,0 +1,10 @@
+//@ check-pass
+// test Intolter::nth_back does not cause UB for ZSTs with high alignment
+
+#[repr(align(8))]
+struct Thing;
+
+fn main() {
+    let v = vec![Thing, Thing];
+    let _ = v.into_iter().nth_back(1);
+}