tests: web-server: Wait until the server is listening.

Fixes synchronization issues observed on slow or loaded machines, where
client connection attempts would fail with ECONNREFUSED:

  https://issues.guix.gnu.org/54348

* test-suite/tests/web-server.test ("server is listening"): New test.

test-suite/tests/web-server.test

@@ -1,6 +1,6 @@
 ;;;; web-server.test --- HTTP server       -*- mode: scheme; coding: utf-8; -*-
 ;;;;
-;;;; 	Copyright (C) 2019, 2020 Free Software Foundation, Inc.
+;;;; 	Copyright (C) 2019-2020, 2022 Free Software Foundation, Inc.
 ;;;;
 ;;;; This library is free software; you can redistribute it and/or
 ;;;; modify it under the terms of the GNU Lesser General Public
@@ -76,6 +76,29 @@
       (throw 'unresolved)))
 
 
+(pass-if "server is listening"
+  ;; First, wait until the server is listening, up to a few seconds.
+  (let ((socket (socket AF_INET SOCK_STREAM 0)))
+    (let loop ((n 1))
+      (define success?
+        (catch 'system-error
+          (lambda ()
+            (format (current-error-port)
+                    "connecting to the server, attempt #~a~%" n)
+            (connect socket AF_INET INADDR_LOOPBACK %port-number)
+            (close-port socket)
+            #t)
+          (lambda args
+            (if (and (= ECONNREFUSED (system-error-errno args))
+                     (<= n 15))
+                #f
+                (apply throw args)))))
+
+      (or success?
+          (begin
+            (sleep 1)
+            (loop (+ n 1)))))))
+
 (pass-if-equal "GET /"
     "Hello, λ world!"
   (expect http-get "/" 200))