Add specs for InstancePolicy (#9587)

spec/policies/instance_policy_spec.rb

+# frozen_string_literal: true
+
+require 'rails_helper'
+require 'pundit/rspec'
+
+RSpec.describe InstancePolicy do
+  let(:subject) { described_class }
+  let(:admin)   { Fabricate(:user, admin: true).account }
+  let(:john)    { Fabricate(:user).account }
+
+  permissions :index?, :resubscribe? do
+    context 'admin' do
+      it 'permits' do
+        expect(subject).to permit(admin, Instance)
+      end
+    end
+
+    context 'not admin' do
+      it 'denies' do
+        expect(subject).to_not permit(john, Instance)
+      end
+    end
+  end
+end