Add spec for QA::Scenario::Entrypoint

For added confidence, and because I plan to fiddle with some behaviours
shortly, add spec testing to the newly extracted
QA::Scenario::Entrypoint class.

qa/spec/scenario/entrypoint_spec.rb

+describe QA::Scenario::Entrypoint do
+  subject do
+    Class.new(QA::Scenario::Entrypoint) do
+      tags :rspec
+    end
+  end
+
+  context '#perform' do
+    let(:config) { spy('Specs::Config') }
+    let(:release) { spy('Runtime::Release') }
+    let(:runner) { spy('Specs::Runner') }
+
+    before do
+      allow(config).to receive(:perform) { |&block| block.call config }
+      allow(runner).to receive(:perform) { |&block| block.call runner }
+
+      stub_const('QA::Specs::Config', config)
+      stub_const('QA::Runtime::Release', release)
+      stub_const('QA::Specs::Runner', runner)
+    end
+
+    it 'should set address' do
+      subject.perform("hello")
+
+      expect(config).to have_received(:address=).with("hello")
+    end
+
+    context 'no paths' do
+      it 'should call runner with default arguments' do
+        subject.perform("test")
+
+        expect(runner).to have_received(:rspec)
+          .with(hash_including(files: 'qa/specs/features'))
+      end
+    end
+
+    context 'specifying paths' do
+      it 'should call runner with paths' do
+        subject.perform('test', 'path1', 'path2')
+
+        expect(runner).to have_received(:rspec)
+          .with(hash_including(files: %w(path1 path2)))
+      end
+    end
+  end
+end