`test_new_error` fails on i686

From [this test job](https://github.com/google/zerocopy/actions/runs/3185233602/jobs/5194573658):

```text
error: test failed, to rerun pass '--lib'
thread 'tests::test_new_error' panicked at 'assertion failed: LayoutVerified::<_, u64>::new(&buf.buf[4..]).is_none()', src/lib.rs:2784:9
```

Fix the bug and re-enable the test in `.github/workflows/ci.yml`.