Turn ASSERTs to CHECKs in test-heap.cc

This fixes unused variable compile failures in release builds introduced in r17887.

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/76413004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17888 ce2b1a6d-e550-0410-aec6-3dcde31c8c00