Tests that make a stack allocated persistent weak and rely on gc need to force gc

Otherwise, if during a fuzzer test incremental marking is running, we'll
crash in the weak callback

BUG=none
TBR=verwaest@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/101843002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18233 ce2b1a6d-e550-0410-aec6-3dcde31c8c00