<script src=”test.js”></script> is correct in HTML5. Self-closed script tag is illegal in html5. The following content after <script src=”test.js”/> will be considered as javascript code until </script> is met. So, the script in other <script> tags after <script src=”test.js”/> may not be executed and the content of the webpage may not be displayed.
The whole <script src=”test.js”></script>, however, can be put inside <head></head>, or <body></body>, even after <body></body>.