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>
diff --git a/Filelist b/Filelist
index d74dc10..b7b3907 100644
--- a/Filelist
+++ b/Filelist
@@ -827,6 +827,7 @@
 		runtime/syntax/testdir/runtest.vim \
 		runtime/syntax/testdir/ftplugin/*.* \
 		runtime/syntax/testdir/input/*.* \
+		runtime/syntax/testdir/input/setup/*.* \
 		runtime/syntax/testdir/dumps/*.dump \
 		runtime/syntax/generator/Makefile \
 		runtime/syntax/generator/README.md \