patch 8.0.1689: no tests for xxd

Problem:    No tests for xxd.
Solution:   Add a test. (Christian Brabandt)
diff --git a/src/testdir/Makefile b/src/testdir/Makefile
index 90e0cdb..459b868 100644
--- a/src/testdir/Makefile
+++ b/src/testdir/Makefile
@@ -4,6 +4,7 @@
 
 # Use console or GUI.
 VIMPROG = ../vim
+XXDPROG = ../xxd/xxd
 # VIMPROG = ../gvim
 
 SCRIPTSOURCE = ../../runtime
@@ -145,3 +146,6 @@
 
 opt_test.vim: ../option.c gen_opt_test.vim
 	$(VIMPROG) -u NONE -S gen_opt_test.vim --noplugin --not-a-term ../option.c
+
+test_xxd.res:
+	XXD=$(XXDPROG); export XXD; $(RUN_VIMTEST) $(NO_INITS) -S runtest.vim test_xxd.vim