[libcxxabi] Cleanup memory in tests to placate ASAN.

Summary: ASAN fires on these tests because they don't clean up their memory.

Reviewers: danalbert, jroelofs, mclow.lists

Reviewed By: jroelofs

Subscribers: dblaikie, cfe-commits

Differential Revision: http://reviews.llvm.org/D6281

llvm-svn: 222493