Add test for checking interchangeability between KubernetesService and…

spec/features/projects/clusters/interchangeability_spec.rb

+require 'spec_helper'
+
+feature 'Interchangeability between KubernetesService and Platform::Kubernetes' do
+  let!(:project) { create(:project, :repository) }
+
+  EXCEPT_METHODS = %i[test title description help fields initialize_properties namespace namespace= api_url api_url=]
+  EXCEPT_METHODS_GREP_V = %w[_touched? _changed? _was]
+
+  it 'Clusters::Platform::Kubernetes covers core interfaces in KubernetesService' do
+    expected_interfaces = KubernetesService.instance_methods(false)
+    expected_interfaces = expected_interfaces - EXCEPT_METHODS
+    EXCEPT_METHODS_GREP_V.each do |g|
+      expected_interfaces = expected_interfaces.grep_v(/#{Regexp.escape(g)}\z/)
+    end
+    
+    expect(expected_interfaces - Clusters::Platforms::Kubernetes.instance_methods).to be_empty
+  end
+
+  shared_examples 'selects kubernetes instance' do
+    context 'when user configured kubernetes from Integration > Kubernetes' do
+      let!(:kubernetes_service) { create(:kubernetes_service, project: project) }
+
+      it { is_expected.to eq(kubernetes_service) }
+    end
+
+    context 'when user configured kubernetes from CI/CD > Clusters' do
+      let!(:cluster) { create(:cluster, :provided_by_gcp, projects: [project]) }
+      let(:platform_kubernetes) { cluster.platform_kubernetes }
+
+      it { is_expected.to eq(platform_kubernetes) }
+    end
+  end
+
+  describe 'Project#deployment_service' do
+    subject { project.deployment_service }
+
+    it_behaves_like 'selects kubernetes instance'
+  end
+
+  describe 'Project#kubernetes_service' do
+    subject { project.kubernetes_service }
+
+    it_behaves_like 'selects kubernetes instance'
+  end
+end