Refactor some allow_value tests

Specifically, tests around the :against option to `with_message`.

* Extract record builder to a separate helper method / class in
  AllowValueMatcherHelpers
* Fix ModelBuilder and ClassBuilder so that they can be required in
  isolation
Refactor validation_confirmation_of tests

Extract record builder code to
builder_for_record_validating_confirmation (located in
Helpers::ConfirmationMatcherHelpers). This includes the name of the
attribute being tested, as this doesn't really matter to our tests.
Fix allow_value + i18n validation messages

Previously, `allow_value` failed when qualified with `with_message` and
the `:against` option. This also happened to propagate downward to
`validate_confirmation_of` since it uses `allow_value` + `:against`
(when a confirmation validation fails, it assigns the error message to
the *_confirmation attribute, not the attribute being confirmed).
This commit fixes both matchers.
Enable caching