tests: suppress message 'Broken pipe exception handling'

Message below is printed every time on shutdown `httpd.py` when
`test/app-luatest/http_client_test.lua` is running by luatest without
capturing stdout:

```
BrokenPipeError: [Errno 32] Broken pipe exception handling
```

The patch suppress this exception by adding a handler for a signal
`SIGPIPE`.

NO_CHANGELOG=testing
NO_DOC=testing
NO_TEST=testing

test/app-luatest/httpd.py

@@ -4,6 +4,9 @@ import sys
 import tempfile
 from gevent.pywsgi import WSGIServer
 from gevent import spawn, sleep, socket
+from signal import signal, SIGPIPE, SIG_DFL
+
+signal(SIGPIPE, SIG_DFL)
 
 def absent():
     code = "500 Server Error"