patch 8.0.1263: others can read the swap file if a user is careless

Problem:    Others can read the swap file if a user is careless with his
            primary group.
Solution:   If the group permission allows for reading but the world
            permissions doesn't, make sure the group is right.
diff --git a/src/Makefile b/src/Makefile
index e55e830..48487aa 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -2259,6 +2259,7 @@
 	test_stat \
 	test_statusline \
 	test_substitute \
+	test_swap \
 	test_syn_attr \
 	test_syntax \
 	test_system \