software/caddy-frontend/test: fix ResourceWarning with unclosed server socket

see commit e6deadaca7f4469ab32e028cdbb029acc9c7d3c3 from slapos.core:

We fork a subprocess to serve requests, but the listening socket is bound
in server constructor and inherited by subprocess. With this pattern, we
have to explicitly close the socket from parent process to prevent the
leak.

software/caddy-frontend/test/test.py

@@ -743,12 +743,15 @@ class HttpFrontendTestCase(SlapOSInstanceTestCase):
     server_process = multiprocessing.Process(
       target=server.serve_forever, name='HTTPServer')
     server_process.start()
+    # from now on, socket is used by server subprocess, we can close it
+    server.socket.close()
     cls.logger.debug('Started process %s' % (server_process,))
 
     cls.backend_https_url = 'https://%s:%s/' % server_https.server_address
     server_https_process = multiprocessing.Process(
       target=server_https.serve_forever, name='HTTPSServer')
     server_https_process.start()
+    server_https.socket.close()
     cls.logger.debug('Started process %s' % (server_https_process,))
 
     class NetlocHandler(TestHandler):
@@ -760,6 +763,7 @@ class HttpFrontendTestCase(SlapOSInstanceTestCase):
     netloc_a_http_process = multiprocessing.Process(
       target=netloc_a_http.serve_forever, name='netloc-a-http')
     netloc_a_http_process.start()
+    netloc_a_http.socket.close()
 
     netloc_b_http = ThreadedHTTPServer(
       (cls._ipv4_address, cls._server_netloc_b_http_port),
@@ -767,6 +771,7 @@ class HttpFrontendTestCase(SlapOSInstanceTestCase):
     netloc_b_http_process = multiprocessing.Process(
       target=netloc_b_http.serve_forever, name='netloc-b-http')
     netloc_b_http_process.start()
+    netloc_b_http.socket.close()
 
     cls.server_process_list = [
       server_process,
@@ -822,6 +827,7 @@ class HttpFrontendTestCase(SlapOSInstanceTestCase):
     self.server_https_auth_process = multiprocessing.Process(
       target=server_https_auth.serve_forever, name='HTTPSServerAuth')
     self.server_https_auth_process.start()
+    server_https_auth.socket.close()
     self.logger.debug('Started process %s' % (self.server_https_auth_process,))
 
   def stopAuthenticatedServerProcess(self):