tests: fixed some 'unused return value' compiler warnings

src/testcurl/https/test_empty_response.c

@@ -181,7 +181,7 @@ testInternalSelectGet ()
             }
         }
       else
-        sleep (1);
+        (void)sleep (1);
       curl_multi_perform (multi, &running);
       if (running == 0)
         {

src/testcurl/https/test_https_get_select.c

@@ -207,7 +207,7 @@ testExternalGet (int flags)
             }
         }
       else
-        sleep (1);
+        (void)sleep (1);
       curl_multi_perform (multi, &running);
       if (running == 0)
         {

src/testcurl/https/test_https_time_out.c

@@ -84,7 +84,7 @@ test_tls_session_time_out (gnutls_session_t session, int port)
       return -1;
     }
 
-  sleep (TIME_OUT + 1);
+  (void)sleep (TIME_OUT + 1);
 
   /* check that server has closed the connection */
   /* TODO better RST trigger */