Fix unused variable warning

We need to call `in_time_zone` to test that it isn't modifying the receiver
but since the variable isn't used it raises a warning so add an assertion
to make Ruby think it's being used.

activesupport/test/core_ext/time_with_zone_test.rb

@@ -1001,6 +1001,7 @@ def test_time_in_time_zone_doesnt_affect_receiver
     with_env_tz 'Europe/London' do
       time = Time.local(2000, 7, 1)
       time_with_zone = time.in_time_zone('Eastern Time (US & Canada)')
+      assert_equal Time.utc(2000, 6, 30, 23, 0, 0), time_with_zone
       assert_not time.utc?, 'time expected to be local, but is UTC'
     end
   end