patch 8.1.0553: it is not easy to edit a script that was sourced

Problem:    It is not easy to edit a script that was sourced.
Solution:   Add a count to ":scriptnames", so that ":script 40" edits the
            script with script ID 40.
diff --git a/src/Make_all.mak b/src/Make_all.mak
index da3a62c..466f17e 100644
--- a/src/Make_all.mak
+++ b/src/Make_all.mak
@@ -153,6 +153,7 @@
 	test_reltime \
 	test_retab \
 	test_ruby \
+	test_scriptnames \
 	test_scroll_opt \
 	test_scrollbind \
 	test_search \