How to configure ocaml ounit tests not to be concurrent?

Question

Currently I have the following for running my unit tests:

let suite = 
  "suite">:::
    ["test_1">:: test_1;
     "test_2">:: test_2;]
;;

let () = 
  run_test_tt_main suite
;;

Problem is, test_1 and test_2 involve some TCP communication, and have shared ports. I need them to run sequentially, I don't care what order.

Edit: command line argument can be specified to run them sequentially

test-name -runner sequential

But how do I make this option the default in the source code?