How to set the variable "app" in Rack Test?

Question

I have an Rack app:

def app
   Rack::Builder.new{
      run MyApp
    }.to_app
end

This works fine, but I want to change app to something else, because there is an model named App.

How to set Rack Test to run against an app with another name?

Working in RSpec. Changing the app results in:

NameError:
   undefined local variable or method `app' for #<RSpec::ExampleGroups::Request:0x007fa76f937ba0>