New test for cc-mode's fontification functions

This issue requires font locking to trigger. This can be be done by
let-binding noninteractive to nil and using a buffer name that doesn't
start with a space, instead of the external emacs hack which is now
redundant.

* yasnippet-tests.el (yas--with-font-locked-temp-buffer): new macro
similar to with-temp-buffer but enables font-lock-mode even in -batch
mode.
(example-for-issue-474): new test
(example-for-issue-404, example-for-issue-404-c-mode): use new macro
(example-for-issue-404-external-emacs): removed
Closes #474: fix cc-mode fontification conflict

* yasnippet.el (yas--save-backquotes): don't change buffer while
narrowed.