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/src/version.c b/src/version.c
index 6d65a6c..56425ab 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1627,
+/**/
     1626,
 /**/
     1625,