TEST: Fix size assertion test for const gen

With const generics we can't avoid the usize (or other type) length
field.

Author: bluss

tests/tests.rs

@@ -279,10 +279,10 @@ fn test_compact_size() {
     println!("{}", mem::size_of::<ByteArray>());
     assert!(mem::size_of::<ByteArray>() <= 8);
 
-    // 1 enum tag + 1 drop flag
+    // just length
     type EmptyArray = ArrayVec<u8,  0>;
     println!("{}", mem::size_of::<EmptyArray>());
-    assert!(mem::size_of::<EmptyArray>() <= 2);
+    assert!(mem::size_of::<EmptyArray>() <= mem::size_of::<usize>());
 
     // 12 element size + 1 enum tag + 3 padding + 1 len + 1 drop flag + 2 padding
     type QuadArray = ArrayVec<u32,  3>;