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/Makefile b/Makefile
index 110a647..6e68093 100644
--- a/Makefile
+++ b/Makefile
@@ -39,14 +39,17 @@
 	@echo "Starting make in the src directory."
 	@echo "If there are problems, cd to the src directory and run make there"
 	cd src && $(MAKE) $@
-	@# When the target is "test" also run the indent tests.
+	@# When the target is "test" also run the indent and syntax tests.
 	@if test "$@" = "test"; then \
 		$(MAKE) indenttest; \
+		$(MAKE) syntaxtest; \
 	fi
-	@# When the target is "clean" also clean for the indent tests.
+	@# When the target is "clean" also clean for the indent and syntax tests.
 	@if test "$@" = "clean" -o "$@" = "distclean" -o "$@" = "testclean"; then \
 		cd runtime/indent && \
 			$(MAKE) clean; \
+		cd runtime/syntax && \
+			$(MAKE) clean; \
 	fi
 
 # Executable used for running the indent tests.
@@ -57,6 +60,14 @@
 		$(MAKE) clean && \
 		$(MAKE) test VIM="$(VIM_FOR_INDENTTEST)"
 
+# Executable used for running the syntax tests.
+VIM_FOR_SYNTAXTEST = ../../src/vim
+
+syntaxtest:
+	cd runtime/syntax && \
+		$(MAKE) clean && \
+		$(MAKE) test VIM="$(VIM_FOR_SYNTAXTEST)"
+
 
 #########################################################################
 # 2. Creating the various distribution files.