runtime(syntax-tests): Retrofit input/c.c. with TEST_SETUP

Also:

- Include a syntax test for Markdown that takes advantage of
  a sourceable setup configuration.
- Update Filelist

closes: #14215

Signed-off-by: Aliaksei Budavei <0x000c70@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/syntax/testdir/input/c.c

@@ -1,4 +1,5 @@
 /* vi:set ts=8 sts=4 sw=4 noet:
+ * TEST_SETUP let g:c_comment_strings = 1
  *
  * VIM - Vi IMproved	by Bram Moolenaar
  *