$ cat resize.d 
#pragma D option quiet

ruby*:::string-capa-resize
{
  printf("string resize from %d to %d (%s:%d)\n", arg0, arg1, copyinstr(arg2), arg3)
}

$ cat my_test.rb
require 'test_helper'

class UsersControllerTest < ActionDispatch::IntegrationTest
  def test_get_new
    get new_user_url
    assert_response :success
  end
end

puts "#" * 90

20.times do
  Minitest.run_one_method UsersControllerTest, :test_get_new
end

exit!

$ sudo dtrace -q -s resize.d -c'/Users/aaron/.rbenv/versions/ruby-trunk/bin/ruby -I lib:test my_test.rb'