Added rspec for ReviewAppSetup file

Added relevant tests for this file.

ee/spec/fixtures/api/schemas/environments.json

@@ -8,6 +8,7 @@
             "minItems": 1,
             "type": "array"
         },
+        "review_app": { "$ref": "review_app.json" },
         "available_count": {
             "type": "integer"
         },

ee/spec/fixtures/api/schemas/review_app.json

+{
+  "type": "object",
+  "required": ["can_setup_review_app?"],
+  "properties": {
+    "can_setup_review_app?": { "type": "boolean" },
+    "clusters_empty?": { "type": "boolean" },
+    "review_snippet": { "type": "string "}
+  },
+  "additionalProperties": false
+}

spec/serializers/review_app_setup_entity_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe ReviewAppSetupEntity do
+  let_it_be(:user) { create(:admin) }
+  let(:project) { create(:project) }
+  let(:presenter) { ProjectPresenter.new(project, current_user: user) }
+  let(:entity) { described_class.new(presenter) }
+  let(:request) { double('request') }
+
+  before do
+    allow(request).to receive(:current_user).and_return(user)
+    allow(request).to receive(:project).and_return(project)
+  end
+
+  subject { entity.as_json }
+
+  describe '#as_json' do
+    it 'contains can_setup_review_app?' do
+      expect(subject).to include(:can_setup_review_app?)
+    end
+
+    context 'when the user can setup a review app' do
+      before do
+        allow(presenter).to receive(:can_setup_review_app?).and_return(true)
+      end
+
+      it 'contains relevant fields' do
+        expect(subject.keys).to include(:clusters_empty?, :review_snippet)
+      end
+
+      it 'exposes the relevant review snippet' do
+        review_app_snippet = YAML.safe_load(File.read(Rails.root.join('lib', 'gitlab', 'ci', 'snippets', 'review_app_default.yml'))).inspect
+
+        expect(subject[:review_snippet]).to eq(review_app_snippet)
+      end
+
+      it 'exposes whether the project has associated clusters' do
+        allow(project).to receive(:clusters).and_return([])
+
+        expect(subject[:clusters_empty?]).to be_truthy
+      end
+    end
+
+    context 'when the user cannot setup a review app' do
+      before do
+        allow(presenter).to receive(:can_setup_review_app?).and_return(false)
+      end
+
+      it 'does not expose certain fields' do
+        expect(subject.keys).not_to include(:clusters_empty?, :review_snippet)
+      end
+    end
+  end
+end