tls: Make test failures more useful

test-tls-server often fails with

    recv_reply: assertion failed (len >= 100): (-1 >= 100)

which is not particularly enlightening. It could be something simple
like EINTR, or the timeout loop actually running all the way to the end
without ever getting an actual response. Make the failure of both of
them more explicit and verbose, and also print errors.

src/tls/test-server.c

@@ -181,6 +181,8 @@ recv_reply (int fd, char *buf, size_t buflen)
 
   len = recv (fd, buf, buflen - 1, MSG_DONTWAIT);
   close (fd);
+  if (len < 0)
+    g_error ("recv_reply: unexpected error: %m");
   g_assert_cmpint (len, >=, 100);
   buf[len] = '\0'; /* so that we can use string functions on it */
 
@@ -192,13 +194,19 @@ do_request (TestCase *tc, const char *request)
 {
   static char buf[4096];
   int fd = do_connect (tc);
+  int res;
 
   send_request (fd, request);
   /* wait until data is available */
-  for (int timeout = 0; timeout < 100 && recv (fd, buf, 100, MSG_PEEK | MSG_DONTWAIT) < 100; ++timeout)
+  for (int timeout = 0; timeout < 100; ++timeout) {
+    res = recv (fd, buf, 100, MSG_PEEK | MSG_DONTWAIT);
+    if (res >= 100)
+      return recv_reply (fd, buf, sizeof (buf));
+
     server_poll_event (100);
+  }
 
-  return recv_reply (fd, buf, sizeof (buf));
+  g_error ("timed out waiting for enough data to become available: res=%d, error: %m", res);
 }
 
 static void