Using RSpec mocks instead of a dynamic module and module prepend

authenticated.rb

# spec/support/authenticated.rb
RSpec.shared_context "authenticated", shared_context: :metadata do
  before do
    allow_any_instance_of(described_class).to receive(:current_user) { current_uset }
  end
end

create_spec.rb

# spec/admin/controllers/study/create_spec.rb
RSpec.describe Admin::Controllers::Study::Create, type: :action do
  let(:action) { described_class.new }
  let(:params) { Hash[] }
  let(:admin) { Fabricate.build(:admin) }

  include_context "authenticated" do
    let(:current_user) { admin }
  end

  it 'is successful' do
    response = action.call(params)
    expect(response[0]).to eq 302
  end
end