Post fix SEMISYNC_PLUGIN_OPT when semi-sync plugins are not found

mysql-test/mysql-test-run.pl:
  Set SEMISYNC_PLUGIN_OPT to '--plugin-dir=' when semi-sync plugins are not found

mysql-test/mysql-test-run.pl

@@ -1836,7 +1836,7 @@ sub environment_setup {
   {
     $ENV{'SEMISYNC_MASTER_PLUGIN'}= "";
     $ENV{'SEMISYNC_SLAVE_PLUGIN'}= "";
-    $ENV{'SEMISYNC_PLUGIN_OPT'}="";
+    $ENV{'SEMISYNC_PLUGIN_OPT'}="--plugin-dir=";
   }
 
   # ----------------------------------------------------