Fix SM-serialization testcase

`xmpp_run_once()` uses `FD_SET()` to determine which sockets to `select()`
on. The socket gets initialized to `INVALID_SOCKET = -1` inside
`xmpp_conn_new()` which leads to a buffer overflow once `FD_SET()` is
called.

This is only exposed when enabling a higher optimization level, which
was only done in the `release-test` CI job.

* Fix this testcase by initializing the socket to a possible value.
* Build the Valgrind CI jobs with `-O2`.
* Make the output of the `release-test` more verbose.

Signed-off-by: Steffen Jaeckel <s@jaeckel.eu>

tests/test_serialize_sm.c

@@ -147,6 +147,7 @@ int main()
     conn->intf = intf;
     state = conn->state;
     conn->state = XMPP_STATE_CONNECTED;
+    conn->sock = 123;
 
     xmpp_send_raw(conn, "foo", 3);
     ENSURE_EQ(xmpp_conn_send_queue_len(conn), 1);