run specs even if Kernel#exit() was called
Created by: sunaku
This commit fixes a problem that I introduced in pull request #569, where RSpec won't run any specs if someone explicitly called exit() or if they were using both Test::Unit and RSpec together and they had loaded RSpec before loading Test::Unit. In particular, exit() called in Test::Unit's at_exit handler would raise SystemExit, thereby defining $!, whose presence inhibits RSpec's at_exit handler from running specs.