`test_as_bytes_methods` fails on powerpc

From [this test job](https://github.com/google/zerocopy/actions/runs/3185927469/jobs/5196030735):

```text
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `[0, 0, 0, 1, 0, 0, 0, 2, 0, 0, 0, 0]`,
 right: `[1, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0]`', src/lib.rs:2885:9
```

Fix the bug and re-enable the test in `.github/workflows/ci.yml`.