Minor typo fix for mysql-test-run.pl

mysql-test/mysql-test-run.pl:
  Fix typo (missing $prefix arg to mtr_add_arg(--user=root))

mysql-test/mysql-test-run.pl

@@ -3765,12 +3765,11 @@ sub mysqld_arguments ($$$$) {
 
 
   # When mysqld is run by a root user(euid is 0), it will fail
-  # to start unless we specify what user to run as. If not running
-  # as root it will be ignored, see BUG#30630
+  # to start unless we specify what user to run as, see BUG#30630
   my $euid= $>;
   if (!$glob_win32 and $euid == 0 and
       grep(/^--user/, @$extra_opt, @opt_extra_mysqld_opt) == 0) {
-    mtr_add_arg($args, "%s--user=root");
+    mtr_add_arg($args, "%s--user=root", $prefix);
   }
 
   if ( $opt_valgrind_mysqld )