netdev-socket test_stream_unix() is unreliable
Description of problem:
test_stream_unix is unreliable and causes random CI job failures such as this one:
https://gitlab.com/qemu-project/qemu/-/jobs/5020899550

```
576/839 ERROR:../tests/qtest/netdev-socket.c:293:test_stream_unix: assertion failed (resp == expect): ("st0: index=0,type=stream,connection error\r\n" == "st0: index=0,type=stream,unix:/tmp/netdev-socket.UW5IA2/stream_unix\r\n") ERROR         
576/839 qemu:qtest+qtest-sh4 / qtest-sh4/netdev-socket                            ERROR          62.85s   killed by signal 6 SIGABRT
>>> MALLOC_PERTURB_=249 QTEST_QEMU_BINARY=./qemu-system-sh4 QTEST_QEMU_STORAGE_DAEMON_BINARY=./storage-daemon/qemu-storage-daemon G_TEST_DBUS_DAEMON=/home/gitlab-runner/builds/-LCfcJ2T/0/qemu-project/qemu/tests/dbus-vmstate-daemon.sh QTEST_QEMU_IMG=./qemu-img /home/gitlab-runner/builds/-LCfcJ2T/0/qemu-project/qemu/build/tests/qtest/netdev-socket --tap -k
――――――――――――――――――――――――――――――――――――― ✀  ―――――――――――――――――――――――――――――――――――――
stderr:
**
ERROR:../tests/qtest/netdev-socket.c:293:test_stream_unix: assertion failed (resp == expect): ("st0: index=0,type=stream,connection error\r\n" == "st0: index=0,type=stream,unix:/tmp/netdev-socket.UW5IA2/stream_unix\r\n")
(test program exited with status code -6)
```