Closes #474: fix more cc-mode font-lock conflicts

* yasnippet-tests.el (example-for-issue-474): Don't call insert from
  within backquotes, it's redundant and prevents proper testing of
  backquote expansion.

* yasnippet.el (yas--restore-backquotes, yas--save-backquotes): Ensure
  the buffer isn't narrowed while modifying it to avoid conflicting with
  cc-mode fontification.