Remove unused AV helper fixtures from e10a2531

Several fixtures for helpers are removed. They were introduced in
ActionView by @strzalek but never referenced in any test.
Bust the template digest cache key when details are changed

Since the lookup details will influence which template is resolved, they
need to be included in the cache key -- otherwise two different
templates may erroneously share the same digest value.
mention controller test base class in testing guide. [ci skip]
Allow attaching to AS::Notifications namespace up front

Before, you were required to attach *after* adding the methods to the
class, since the attachment process needed the methods to be present.

With this change, any new method will also be attached to the configured
namespace.
Fix: Remove broken link on rails 3 guide

As raised in https://github.com/rails/rails/issues/12300
[ci skip]