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/testdir/Make_all.mak b/src/testdir/Make_all.mak
index 072c61c..63a1b05 100644
--- a/src/testdir/Make_all.mak
+++ b/src/testdir/Make_all.mak
@@ -184,6 +184,7 @@
 	    test_stat.res \
 	    test_substitute.res \
 	    test_syntax.res \
+	    test_system.res \
 	    test_textobjects.res \
 	    test_undo.res \
 	    test_usercommands.res \