Detect fin state of a QUIC stream for streams which are completely read

SSL_poll indicates that a stream which has had the fin bit set on it,
should generate SSL_POLL_EVENT_R events, so that applications can detect
stream completion via SSL_read_ex and SSL_get_error returning
SSL_ERROR_ZERO_RETURN.

However, the quic polling code misses on this, as a client that
completely reads a buffer after receipt has its underlying stream buffer
freed, loosing the fin status

We can however detect stream completion still, as a stream which has
been finalized, and had all its data read will be in the
QUIC_RSTREAM_STATE_DATA_READ state, iff the fin bit was set.

Fix it by checking in test_poll_event_r for that state, and generating a
SSL_POLL_EVENT_R if its found to be true, so as to stay in line with the
docs.

Fixes openssl/private#627

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Sasa Nedvedicky <sashan@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/25399)