src/template/variable-scope.php: Make exception to NOT re-scope special $this variable.
test/: Use 'diet-php' as the extension for our template files, instead of 'sphp'.
test/template/inheritance/: Remove extend-php-class.sphp for now, as it's not being used at moment and is leading to warnings when running test suite.
test/template/inheritance/test.php: Re-enable proper implementation of 'testBasicInheritance' (now passing), and comment-out and rename function that was being used for debugging.