Add specs for model facebook_page (#346)

spec/models/channel/facebook_page_spec.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe Channel::FacebookPage do
+  before { create(:channel_facebook_page) }
+
+  it { is_expected.to validate_presence_of(:account_id) }
+  it { is_expected.to validate_uniqueness_of(:page_id).scoped_to(:account_id) }
+  it { is_expected.to belong_to(:account) }
+  it { is_expected.to have_one(:inbox).dependent(:destroy) }
+end