mtr: use process launch -- args to start mysqld in lldb

gdb's "set args" equivalent for lldb would be "settings set target.run-args",
however it doesn't play well with double dashed args (--).

mysql-test/mysql-test-run.pl

@@ -6051,7 +6051,7 @@ sub lldb_arguments {
   $input = $input ? "< $input" : "";
 
   # write init file for mysqld or client
-  mtr_tofile($lldb_init_file, "set args $str $input\n");
+  mtr_tofile($lldb_init_file, "process launch --stop-at-entry -- $str $input\n");
 
     print "\nTo start lldb for $type, type in another window:\n";
     print "cd $glob_mysql_test_dir && lldb -s $lldb_init_file $$exe\n";