Random failures in TestServerHandshakeRefuseVersionMismatch

We get this test failure sporadically. It's probably because we're closing the test connection too early.

```
=== RUN   TestServerHandshakeRefuseVersionMismatch
    server_test.go:153: unexpected error when creating Ouroboros object: could not register protocol with muxer
--- FAIL: TestServerHandshakeRefuseVersionMismatch (0.00s)
```