[WIP] More tests for TLSPlaintext record parser

Some negative tests.

This needs to be updated after @radix's #5 is merged.
You'll want to write this as:

```py
with pytest.raises(ValueError) as exc_info:
    parse_tls_plaintext(packet)
assert exc_info.value.message == "..."
```

Otherwise it will pass if no exception is raised.

Also, I"m pretty sure `e.message` doesn't exist on py3k.