Cleanup tests for StringCharacterStream

Edge case tests now cover all branches.

R=yangguo@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/11548023
Patch from Dan Carney <dcarney@google.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13217 ce2b1a6d-e550-0410-aec6-3dcde31c8c00