test_pack.rb: test_invalid_warning

* test/ruby/test_pack.rb (test_invalid_warning): add test for
  invalid directive warnings.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48306 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_pack.rb

@@ -714,4 +714,9 @@ EXPECTED
     $VERBOSE = verbose
   end
 
+  def test_invalid_warning
+    assert_warning(/unknown pack directive ',' in ','/) {
+      [].pack(",")
+    }
+  end
 end