Flaky tests

## Bug description
During a recent merge, we noticed that suddenly coverage went down in a part of the library that we hadn't touched. After some examination, we discovered that these two tests are flaky:

This test is flaky around 20% of the time:
`test_wordlist_codec_error_detection`

This one is flaky about 5% of the time:
`test_wordlist_codec_different_data_different_encodings`

To prove this, you can run:
```
$ while true; do; cargo test --features bip39,bip39-en,bip39-jp --test bip39; done
```

These two tests have something in common:
```
OsRng.fill_bytes(&mut data);
let ws = choose_wordlist();
let ms = wordlist::encode(&data, ws).unwrap();
```