Add spec for migration warning module (#25033)

spec/lib/mastodon/migration_warning_spec.rb

+# frozen_string_literal: true
+
+require 'rails_helper'
+require 'mastodon/migration_warning'
+
+describe Mastodon::MigrationWarning do
+  describe 'migration_duration_warning' do
+    before do
+      allow(migration).to receive(:valid_environment?).and_return(true)
+      allow(migration).to receive(:sleep).with(1)
+    end
+
+    let(:migration) { Class.new(ActiveRecord::Migration[6.1]).extend(described_class) }
+
+    context 'with the default message' do
+      it 'warns about long migrations' do
+        expectation = expect { migration.migration_duration_warning }
+
+        expectation.to output(/interrupt this migration/).to_stdout
+        expectation.to output(/Continuing in 5/).to_stdout
+      end
+    end
+
+    context 'with an additional message' do
+      it 'warns about long migrations' do
+        expectation = expect { migration.migration_duration_warning('Get ready for it') }
+
+        expectation.to output(/interrupt this migration/).to_stdout
+        expectation.to output(/Get ready for it/).to_stdout
+        expectation.to output(/Continuing in 5/).to_stdout
+      end
+    end
+  end
+end