Tightened the let and const tests

This changes the ES6 let and const tests, requiring them to implement the "temporal dead zone", the for-let loop's fresh bindings specified in [rev 23](http://wiki.ecmascript.org/doku.php?id=harmony:specification_drafts#april_5_2014_draft_rev_23), and thrown syntax errors for incorrect const usage. Also, strict mode ("extended mode") and the "application/javascript;version=1.8" attribute are no longer added to the test.

Didn't get around to testing if IE11 or EJS passes these altered tests.