patch 9.0.1627: no generic mechanism to test syntax plugins

Problem:    No generic mechanism to test syntax plugins.
Solution:   Add a syntax plugin test mechanism, using screendumps.  Add a
            simple test for "c".
diff --git a/Filelist b/Filelist
index 392ef80..c6c53e7 100644
--- a/Filelist
+++ b/Filelist
@@ -805,6 +805,11 @@
 		runtime/syntax/README.txt \
 		runtime/syntax/shared/*.vim \
 		runtime/syntax/shared/README.txt \
+		runtime/syntax/Makefile \
+		runtime/syntax/testdir/README.txt \
+		runtime/syntax/testdir/runtest.vim \
+		runtime/syntax/testdir/input/*.* \
+		runtime/syntax/testdir/dumps/*.dump \
 
 # Unix runtime
 RT_UNIX =	\