Adds spec for destroyable field

spec/requests/api/graphql/ci/pipeline_spec.rb

+# frozen_string_literal: true
+require 'spec_helper'
+
+RSpec.describe 'destroyable' do
+  include GraphqlHelpers
+
+  let_it_be(:project) { create(:project) }
+  let_it_be(:user) { create(:user) }
+  let_it_be(:project_owner) { project.owner }
+  let_it_be(:pipeline) { create(:ci_pipeline, project: project) }
+
+  let(:query) do
+    %(
+      query {
+        project(fullPath: "#{project.full_path}") {
+          pipeline(iid: "#{pipeline.iid}") {
+            destroyable
+          }
+        }
+      }
+    )
+  end
+
+  let(:pipeline_graphql_data) { graphql_data.dig('project', 'pipeline', 'destroyable') }
+
+  context 'when user does not have the permission' do
+    before do
+      project.add_developer(user)
+      post_graphql(query, current_user: user)
+    end
+
+    it 'returns false' do
+      allow(Ability).to receive(:allowed?).with(user, :destroy_pipline, pipeline).and_return(false)
+
+      expect(pipeline_graphql_data).to be(false)
+    end
+  end
+
+  context 'when user is owner and therefore has the permission' do
+    before do
+      post_graphql(query, current_user: project_owner)
+    end
+
+    it 'returns true' do
+      allow(Ability).to receive(:allowed?).with(project_owner, :destroy_pipline, pipeline).and_return(true)
+
+      expect(pipeline_graphql_data).to be(true)
+    end
+  end
+end