TEST: Update tests for new MaybeUninit usage

(Also remove the size check since we can't rely on it, even if we want
to ensure it has no overhead for the bytes case.)

tests/tests.rs

@@ -510,10 +510,8 @@ fn test_sizes_129_255() {
 
 
 #[test]
-fn test_nightly_uses_maybe_uninit() {
-    if option_env!("ARRAYVECTEST_ENSURE_UNION").map(|s| !s.is_empty()).unwrap_or(false) {
-        assert!(cfg!(has_manually_drop_in_union));
-        type ByteArray = ArrayVec<[u8; 4]>;
-        assert!(mem::size_of::<ByteArray>() == 5);
+fn test_newish_stable_uses_maybe_uninit() {
+    if option_env!("ARRAYVECTEST_ENSURE_MAYBEUNINIT").map(|s| !s.is_empty()).unwrap_or(false) {
+        assert!(cfg!(has_stable_maybe_uninit));
     }
 }