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/testdir/Make_all.mak b/src/testdir/Make_all.mak
index 45f45d7..5b85156 100644
--- a/src/testdir/Make_all.mak
+++ b/src/testdir/Make_all.mak
@@ -158,6 +158,7 @@
 	    test_registers.res \
 	    test_retab.res \
 	    test_ruby.res \
+	    test_scriptnames.res \
 	    test_scrollbind.res \
 	    test_search.res \
 	    test_shortpathname.res \