Fix a failing test in Railtie and properly define all severity levels in MockLogger for LogSubscriber.

Move Rails::LogSubscriber to ActiveSupport::LogSubscriber, allowing frameworks like ActiveRecord and ActiveResource to log outsude Rails::Application [#4816 state:resolved]

Have log subscribers subscribe to the actual events, so the subscriber doesn't subscribe to *every* event, so we can have events that are slow-ish but are not actually run in production.

Create Rails::Subscriber::TestHelper and use it to make ActiveRecord subscriber tests run in both sync and async scenarios.