How can I test a code with 'Thread.new' in Rails?

In my rails app, Signup Class has below functions,

def register_email
  # Something...
  add_to_other_thread do
    send_verification_email
  end
end

def add_to_other_thread(&block)
  Thread.new do
    yield
    ActiveRecord::Base.connection.close
  end
end

And I want to do 3 tests with these.

1. Test about add_to_other_thread(&blcok):
   After add_to_other_thread being called with some block, Check whether or not it called Thread.new with proper block.
2. Test about register_email:
   After register_email being called, Check whether add_to_other_thread(&block) got proper block or not.
3. In Integral Test:
   After User signing up, Check whether or not proper email was sent(with ActionMailer::Base.deliveries) via other Thread.