add 2 assertions.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@6059 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_time.rb

@@ -65,7 +65,9 @@ class TestTime < Test::Unit::TestCase
   def test_huge_difference # [ruby-dev:22619]
     if negative_time_t?
       assert_equal(Time.at(-0x80000000), Time.at(0x7fffffff) - 0xffffffff)
+      assert_equal(Time.at(-0x80000000), Time.at(0x7fffffff) + (-0xffffffff))
       assert_equal(Time.at(0x7fffffff), Time.at(-0x80000000) + 0xffffffff)
+      assert_equal(Time.at(0x7fffffff), Time.at(-0x80000000) - (-0xffffffff))
     end
   end
 end