Add specs for a simplifiable CI/CD entry aspect

spec/lib/gitlab/ci/config/entry/simplifiable_spec.rb

+require 'spec_helper'
+
+describe Gitlab::Ci::Config::Entry::Simplifiable do
+  describe '.strategy' do
+    let(:entry) do
+      Class.new(described_class) do
+        strategy :Something, if: -> { 'condition' }
+        strategy :DifferentOne, if: -> { 'condition' }
+      end
+    end
+
+    it 'defines entry strategies' do
+      expect(entry.strategies.size).to eq 2
+      expect(entry.strategies.map(&:name))
+        .to eq %i[Something DifferentOne]
+    end
+  end
+
+  describe 'setting strategy by a condition' do
+    let(:first) { double('first strategy') }
+    let(:second) { double('second strategy') }
+    let(:unknown) { double('unknown strategy') }
+
+    before do
+      stub_const("#{described_class.name}::Something", first)
+      stub_const("#{described_class.name}::DifferentOne", second)
+      stub_const("#{described_class.name}::UnknownStrategy", unknown)
+    end
+
+    context 'when first strategy should be used' do
+      let(:entry) do
+        Class.new(described_class) do
+          strategy :Something, if: -> (arg) { arg == 'something' }
+          strategy :DifferentOne, if: -> (*) { false }
+        end
+      end
+
+      it 'attemps to load a first strategy' do
+        expect(first).to receive(:new).with('something', anything)
+
+        entry.new('something')
+      end
+    end
+
+    context 'when second strategy should be used' do
+      let(:entry) do
+        Class.new(described_class) do
+          strategy :Something, if: -> (arg) { arg == 'something' }
+          strategy :DifferentOne, if: -> (arg) { arg == 'test' }
+        end
+      end
+
+      it 'attemps to load a second strategy' do
+        expect(second).to receive(:new).with('test', anything)
+
+        entry.new('test')
+      end
+    end
+
+    context 'when neither one is a valid strategy' do
+      let(:entry) do
+        Class.new(described_class) do
+          strategy :Something, if: -> (*) { false }
+          strategy :DifferentOne, if: -> (*) { false }
+        end
+      end
+
+      it 'instantiates an unknown strategy' do
+        expect(unknown).to receive(:new).with('test', anything)
+
+        entry.new('test')
+      end
+    end
+  end
+end