test_timeout.rb: add an assertion

* test/test_timeout.rb (test_custom_exception): assert that the
  given exception will raise on timeout.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51214 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/test_timeout.rb

@@ -60,6 +60,11 @@ class TestTimeout < Test::Unit::TestCase
     assert_nothing_raised(ArgumentError, bug9354) do
       assert_equal(:ok, timeout(100, err) {:ok})
     end
+    assert_raise_with_message(err, /execution expired/) do
+      timeout 0.01, err do
+        sleep 3
+      end
+    end
   end
 
   def test_exit_exception