Use BOOST_SCOPE_EXIT to clean up after tests that change global state

[daira]

If a test modifies global state and then doesn't reach the cleanup code because of an exception, that can be exceptionally (haha) annoying when debugging tests.

BOOST_SCOPE_EXIT uses RAII to simulate the try..finally construct from Java and other languages. Note that it needs to go just before the code that modifies the global state.