Fix flaky failures in client auth failure tests

The test is currently too strict about which errors to expect after the server drops the connection. It expects G_IO_ERROR_BROKEN_PIPE to only occur during the close, but it could also occur during the read, in which case there will be no error on close. It's a shame we can't guarantee a consistent error in this case and the errors in general could stand to be improved, but important thing is that there is some error: the read is (properly) not working.

Fixes #66 (closed)