title: How to access txt file using relative path
body: <p>I'm currently trying to open a file using 
  io.open(file_name, "r")</p>
  
  <p>My directory structure is</p>
  
  <pre><code>parent_dir  
     -src  
        -main.lua  
     -tests
        -test.txt
  </code></pre>
  
  <p>I'm currently trying to open the txt file from the tests folder in the main.lua using 
  <code>io.open(file_name, "r")</code> but it won't recognize the file as existing. If I place the absolute path into the file_name it works. I'm not sure how to format a relative path? I've tried some version of <code>../../../tests/test.txt</code> but I can't seem to get it to work.
  The actual path of the file is something like <code>/Users/user_name/parent_dir/tests/test.txt</code></p>