Increase code coverage for admin/cohorts page

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

spec/features/admin/admin_cohorts_spec.rb

+require 'rails_helper'
+
+feature 'Admin cohorts page', feature: true do
+  before do
+    login_as :admin
+  end
+
+  scenario 'See users count per month' do
+    2.times { create(:user) }
+
+    visit admin_cohorts_path
+
+    expect(page).to have_content("#{Time.now.strftime('%b %Y')} 3 0")
+  end
+end