fix: railtie to use correct load hook

[bf4]

I was getting a FrozenError in my app testing v0-11-dev
and noticed that the problem was in the JR initializer call
to ActionDispatch::IntegrationTest

per

• https://github.com/rails/rails/blob/6-0-stable/actionpack/lib/action_dispatch/testing/integration.rb
• https://guides.rubyonrails.org/v6.0/engines.html#available-hooks

Load hooks is the recommended way to add this behavior.

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[bf4]

@lgebhardt this should have used this load hook