write_read only tests utf8 encoding

The test function [`write_read`](https://github.com/haskell/text/blob/f4c2b7ebf3443c0e5ee23a15c6409ac8c83d8b49/tests/Tests/QuickCheckUtils.hs#L243) does not very the encoding.

This seems like an important parameter to very to sure that other encodings work properly.