Fixed benchmark lib require without inclusion in $LOADED_FEATURES

Hello,

`TestGemRequire#test_activate_via_require_respects_loaded_files` requires the benchmark library but removes the path from `$LOADED_FEATURES`, therefore making it possible to be required twice (and so causing redefined constants warnings and such).

This patch requires `'benchmark'` before the `save_loaded_features` block to avoid such problem. An easy way to see the problem is to add `require 'benchmark'` in a test executed after this one.