Write PHP tests

[benlk]

• fab wp.tests_setup:pym
• review code for things that will need to be tested especially
• create list of high-priority tests and low-priority tests
  • assume that WordPress settings functions work
• Write tests for pym_shortcode( $atts, $context, $tag )
  • alternate src is included if set
  • otherwise, src is the path in the plugins directory plugins_url( '/js/pym.js',$whatever_the_pym_plugin_file_path_is )
  • page contains markup corresponding to placed source code
  • a post with n shortcodes contains each of echo '<div id="pym_' . $n . '"></div>';