patch 8.0.0151: passing buffer content to system() is clumsy

Problem:    To pass buffer content to system() and systemlist() one has to
            first create a string or list.
Solution:   Allow passing a buffer number. (LemonBoy, closes #1240)
diff --git a/src/Makefile b/src/Makefile
index 4b29ae1..3f61de6 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -2164,6 +2164,7 @@
 	test_substitute \
 	test_syn_attr \
 	test_syntax \
+	test_system \
 	test_tabline \
 	test_tabpage \
 	test_tagcase \