Make math-round.js test check both normal and optimized version.
Add some cases to the tests.

BUG=v8:958
TEST=mjsunit/math-round

Review URL: http://codereview.chromium.org/6837018

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7599 ce2b1a6d-e550-0410-aec6-3dcde31c8c00