Require 'uri' explicitly to address NameError: uninitialized constant ToQueryTest::URI

This commit addresses CI failure
at https://buildkite.com/rails/rails-nightly/builds/108#018d57ac-4f2a-45f1-86b9-9015a7b0a463/1165-2002

* Error addressed by this commit
```
$ ruby -v
ruby 3.4.0dev (2024-01-30T10:19:23Z master 86547fd69d) [x86_64-linux]
$ rm Gemfile.lock
$ bundle install
$ cd activesupport
$ bin/test test/core_ext/object/to_query_test.rb -n test_hash_not_sorted_lexicographically_for_nested_structure
... snip ...

E

Error:
ToQueryTest#test_hash_not_sorted_lexicographically_for_nested_structure:
NameError: uninitialized constant ToQueryTest::URI
    test/core_ext/object/to_query_test.rb:90:in `test_hash_not_sorted_lexicographically_for_nested_structure'

bin/test test/core_ext/object/to_query_test.rb:79

Finished in 0.000385s, 2597.3082 runs/s, 0.0000 assertions/s.
1 runs, 0 assertions, 0 failures, 1 errors, 0 skips
$
```

This behavior has been introduced since Ruby 3.4.0dev vendors URI from top level to `Gem::URI`
via ruby/ruby@d64d0b5 .
Here is the RubyGems one.
rubygems/rubygems#7386

Author: yahonda

activesupport/test/core_ext/object/to_query_test.rb

@@ -3,6 +3,7 @@
 require_relative "../../abstract_unit"
 require "active_support/core_ext/object/to_query"
 require "active_support/core_ext/string/output_safety"
+require "uri"
 
 class ToQueryTest < ActiveSupport::TestCase
   def test_simple_conversion