Add missing ostruct require on AR test suite

Similar to 50515fb, make sure we
require `ostruct` where we use `OpenStruct`, to get the build back to
green, while we work to remove its usage on tests. (see rails#51510.)

Sample error:

```
Error:
ActiveRecord::Encryption::ConfigurableTest#test_installing_autofiltered_parameters_will_add_the_encrypted_attribute_as_a_filter_parameter_using_the_dot_notation:
NameError: uninitialized constant ActiveRecord::Encryption::ConfigurableTest::OpenStruct
    test/cases/encryption/configurable_test.rb:45:in `block in <class:ConfigurableTest>'
```

activerecord/test/cases/encryption/configurable_test.rb

@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 
+require "ostruct"
 require "cases/encryption/helper"
 require "models/pirate"
 require "models/book"