a.rb

class MyHTTP
  def get(uri)
    @response = HTTP.get(uri)
  end
  
  def response
    @response
  end
end

describe MyHTTP do
  let(:http) { MyHTTP.new }
  
  context 'when retrieving response' do
    subject { http.response }
    
    context 'after getting uri' do
      before { http.get('/') }
      it { is_expected.not_to be_null }
    end
  
    context 'without getting' do
      it { is_expected.to be_null }
    end
  end
end

b.rb

class MySimpleHTTP
  def get(uri)
    HTTP.get(uri)
  end
end

describe MySimpleHTTP do
  let(:http) { MySimpleHTTP.new }
  
  context 'when getting uri' do
    subject { http.get('/') }
    it { is_expected.not_to be_null }
  end
end