Fix test with rack test 2

[rchekaluk]

As discussed in https://github.com/rails/rails/issues/45467, when using gem rack-test '>= 2.0.0', running tests throws the following error:

Error:
ActionControllerTest#test_getting_session_value_after_session_reset:
NoMethodError: undefined method `hash_for' for #<Rack::Test::CookieJar:0x00007fb23bbd7aa0 @default_host="www.example.com", @cookies=[#<Rack::Test::Cookie:0x00007fb23b3b7e10 @default_host="www.example.com", @raw="_session_id=1da458b0817c79f5e30b20428e206e1b", @name="_session_id", @value="1da458b0817c79f5e30b20428e206e1b", @options={"path"=>"/", "HttpOnly"=>nil, "domain"=>"www.example.com"}, @exact_domain_match=true>]>

      session_cookie = cookies.send(:hash_for)['_session_id']
                              ^^^^^
    /REDACTED/activerecord-session_store/test/action_controller_test.rb:130:in `block in test_getting_session_value_after_session_reset'
    /REDACTED/activerecord-session_store/test/helper.rb:53:in `block in with_test_route_set'
    /REDACTED/.rvm/gems/ruby-3.1.2/gems/actionpack-7.0.4/lib/action_dispatch/testing/assertions/routing.rb:174:in `with_routing'
    /REDACTED/activerecord-session_store/test/helper.rb:43:in `with_test_route_set'
    /REDACTED/activerecord-session_store/test/action_controller_test.rb:126:in `test_getting_session_value_after_session_reset'

Emulating Rails fixes in https://github.com/rails/rails/pull/45534, this PR changes a single use of hash_for to get_cookie