(bug 38152) jquery.tablesorter: Use .data() instead of .attr()

(bug 38152) jquery.tablesorter: Use .data() instead of .attr()

* .data() is faster when looking up repeatedly

* .data() will return the latest value, even if it has changed over
   time, whereas .attr() only gives the value as it was when the page
   was generated. Much like the difference between attr() and prop()
   when it comes to value, checked, disabled etc.

* Moved buildCache() call outside `if ( firstTime )`.
  Tests show that this is fast enough to do on the fly. Left inline
  comment with proposal on how to make this more elaborate in the
  future would it become necessary.

* jquery.tablesorter.test.js:
 - Add tests for bug 38152
 - Clean up double quotes
 - Use more descriptive id (prefix with mw-bug-)

Change-Id: Ie4f801c5b1f93617fd3fd173d2eaaf173a7604e9