ci: syntax tests spam output

Currently syntax tests outputs all the Vim commands to the console,
which is annoying for a local developer but also makes the CI output
impossible to parse and just printing all the terminal output comes with
a performance penalty.

So let's just simply redirect all output to /dev/null similar to what
the script tests do.

This can be turned off locally for someone diagnosing issues but it
should not be the default behavior to output all terminal control
sequences to output in CI.

closes: #16612

Signed-off-by: Yee Cheng Chin <ychin.git@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

Author: ychin

runtime/syntax/Makefile

@@ -26,11 +26,12 @@ RUN_VIMTEST = VIMRUNTIME=$(VIMRUNTIME) $(VALGRIND) $(ENVVARS) ../$(VIMPROG) -f $
 # If a test succeeds a testdir/done/{name} file will be written.
 # If a test fails a testdir/failed/{name}.dump file will be written.
 # Progress and error messages can be found in "testdir/messages".
+# Comment out the "> /dev/null" part to see the in-progress Vim behaviors.
 test:
 	@# the "vimcmd" file is used by the screendump utils
 	@echo "../$(VIMPROG)" > testdir/vimcmd
 	@echo "$(RUN_VIMTEST)" >> testdir/vimcmd
-	VIMRUNTIME=$(VIMRUNTIME) $(ENVVARS) $(VIMPROG) --clean --not-a-term $(DEBUGLOG) -u testdir/runtest.vim
+	VIMRUNTIME=$(VIMRUNTIME) $(ENVVARS) $(VIMPROG) --clean --not-a-term $(DEBUGLOG) -u testdir/runtest.vim > /dev/null 
 	@rm -f testdir/Xfilter
 	@# FIXME: Temporarily show the whole file to find out what goes wrong
 	@#if [ -f testdir/messages ]; then tail -n 6 testdir/messages; fi