patch 8.0.1689: no tests for xxd

Problem:    No tests for xxd.
Solution:   Add a test. (Christian Brabandt)
diff --git a/src/Makefile b/src/Makefile
index e7d0b38..2a2940e 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -2308,6 +2308,7 @@
 	test_windows_home \
 	test_wordcount \
 	test_writefile \
+	test_xxd \
 	test_alot_latin \
 	test_alot_utf8 \
 	test_alot: