More cctest fixes regarding lifetime/ownership.

   * Avoid double-free in cctest/test-api/RegExpInterruption.

   * Avoid incorrect zone fiddling in cctest/test-strings.

R=ulan@chromium.org

Review URL: https://codereview.chromium.org/144533005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19020 ce2b1a6d-e550-0410-aec6-3dcde31c8c00