c++: Add test for vt/ff in line comments

c++: Add test for vt/ff in line comments

P2843R3 dropped the
"If there is a form-feed or a vertical-tab character in such a comment, only
whitespace characters shall appear between it and the new-line that terminates
the comment; no diagnostic is required."
sentence from [lex.comment].  AFAIK we've never diagnosed nor checked for
that and C23 doesn't have anything like that, so the following testcase
merely tests that we don't diagnose anything on it.

2025-08-06  Jakub Jelinek  <jakub@redhat.com>

	PR preprocessor/120778
	* c-c++-common/cpp/comment-ff-1.c: New test.
	* c-c++-common/cpp/comment-vtab-1.c: New test.