Rename drupal_autoload_test_trait.sh to use a standard extension once the testbot allows it

See #2508055: Add support for autoloading Traits for discussion.

The patch there adds modules/simpletest/tests/drupal_autoload_test/drupal_autoload_test_trait.sh as a workaround to allow a PHP 5.4 file into the Drupal 7 tests while preventing PHP < 5.4 testbots from gagging on it while running a syntax check.

If the testbots ever change to be a bit more nuanced about what files they run a syntax check on, we could rename the file and remove the @todo.

Ideally we would keep using an extension that .htaccess blocks though (which is why .sh was originally chosen) to prevent flooding a PHP < 5.4 site with errors by visiting this file.