Add missing add index rubocop spec

changelogs/unreleased/add_missing_add_index_rubocop_spec.yml

+---
+title: add missing test for add_index rubocop rule
+merge_request: 20464
+author: Eric Thomas
+type: other

spec/rubocop/cop/migration/add_index_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+require 'rubocop'
+require 'rubocop/rspec/support'
+
+require_relative '../../../../rubocop/cop/migration/add_index'
+
+describe RuboCop::Cop::Migration::AddIndex do
+  include CopHelper
+
+  subject(:cop) { described_class.new }
+
+  context 'in migration' do
+    before do
+      allow(cop).to receive(:in_migration?).and_return(true)
+    end
+
+    it 'registers an offense when add_index is used' do
+      expect_offense(<<~PATTERN.strip_indent)
+        def change
+          add_index :table, :column
+          ^^^^^^^^^ `add_index` requires downtime, use `add_concurrent_index` instead
+        end
+      PATTERN
+    end
+  end
+
+  context 'outside of migration' do
+    it 'registers no offense' do
+      expect_no_offenses(<<~PATTERN.strip_indent)
+        def change
+          add_index :table, :column
+        end
+      PATTERN
+    end
+  end
+end