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>

.gitignore

@@ -46,6 +46,7 @@ examples/roster
 examples/uuid
 examples/vcard
 testbuild*.log
+testerr*.log
 test-release/
 test_stamp
 test-suite*.log