Tune cctest/test-parsing/ParserSync.

This test suffers heavily from combinatorial explosion. To redemedy
this situation a bit, two independent parsing flags are tested
separately, reducing the runtime by a factor of 4. Furthermore, the
code has been cleaned up a bit.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/40113002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17385 ce2b1a6d-e550-0410-aec6-3dcde31c8c00