patch 8.0.1689: no tests for xxd

Problem:    No tests for xxd.
Solution:   Add a test. (Christian Brabandt)
diff --git a/src/testdir/Make_all.mak b/src/testdir/Make_all.mak
index b0bfe26..e36089a 100644
--- a/src/testdir/Make_all.mak
+++ b/src/testdir/Make_all.mak
@@ -188,6 +188,7 @@
 	    test_windows_home.res \
 	    test_wordcount.res \
 	    test_writefile.res \
+	    test_xxd.res \
 	    test_alot_latin.res \
 	    test_alot_utf8.res \
 	    test_alot.res