At the moment svnserve tries to read the config file even if you use 
'svnserve -t'.

this patch passes the run_mode to find_repos(). if we run in tunnel
mode it skips reading the password file.

Index: serve.c
===================================================================
--- serve.c	(revision 16808)
+++ serve.c	(working copy)
@@ -2009,8 +2009,11 @@
   /* Read repository configuration. */
   SVN_ERR(svn_config_read(&b->cfg, svn_repos_svnserve_conf(b->repos, pool),
                           FALSE, pool));
-  svn_config_get(b->cfg, &pwdb_path, SVN_CONFIG_SECTION_GENERAL,
-                 SVN_CONFIG_OPTION_PASSWORD_DB, NULL);
+  if (!b->tunnel)
+    {
+      svn_config_get(b->cfg, &pwdb_path, SVN_CONFIG_SECTION_GENERAL,
+                     SVN_CONFIG_OPTION_PASSWORD_DB, NULL);
+    }
   if (pwdb_path)
     {
       pwdb_path = svn_path_join(svn_repos_conf_dir(b->repos, pool),