tests: CI output is too noisy

Problem:  tests: CI output is too noisy and shows all translated messages
          strings
Solution: redirect stdout of check.vim to /dev/null

Signed-off-by: Christian Brabandt <[email protected]>

src/po/Makefile

@@ -41,7 +41,7 @@ converted: $(MOCONVERTED)
 
 .po.ck:
 	$(VIMPROG) -u NONE --noplugins -e -s -X --cmd "set enc=utf-8" \
-		-S check.vim -c "if error == 0 | q | else | num 2 | cq | endif" $<
+		-S check.vim -c "if error == 0 | q | else | num 2 | cq | endif" $< >/dev/null
 	touch $@
 
 check: $(CHECKFILES)