Move and update enlist_fixture_connections test

This moves the previous test into the old test and reuses the connection
that that test establishes rather than requiring we muck with temporary
connection pool.

The change here is more correct than the previous code since we're
establishing new connections we should be checking the newly established
reading and writing connections are the same, not checking against the
existing ActiveRecord::Base.connection.

The test here also most closely emulates a real application using
multiple databases.

Co-authored-by: John Crepezzi <john.crepezzi@gmail.com>

activerecord/test/cases/fixtures_test.rb

@@ -1391,37 +1391,8 @@ def self.file_fixture_path
   end
 end
 
-class MultipleDatabaseFixturesTest < ActiveRecord::TestCase
-  test "enlist_fixture_connections ensures multiple databases share a connection pool" do
-    old_handlers = ActiveRecord::Base.connection_handlers
-    ActiveRecord::Base.connection_handlers = {}
-
-    with_temporary_connection_pool do
-      ActiveRecord::Base.connects_to database: { writing: :arunit, reading: :arunit2 }
-
-      rw_conn = ActiveRecord::Base.connection
-      ro_conn = ActiveRecord::Base.connection_handlers[:reading].connection_pool_list.first.connection
-
-      assert_equal rw_conn, ro_conn
-    end
-  ensure
-    ActiveRecord::Base.connection_handlers = old_handlers
-  end
-
-  private
-    def with_temporary_connection_pool
-      pool_config = ActiveRecord::Base.connection_handler.send(:owner_to_pool_manager).fetch("ActiveRecord::Base").get_pool_config(:default)
-      new_pool = ActiveRecord::ConnectionAdapters::ConnectionPool.new(pool_config)
-
-      pool_config.stub(:pool, new_pool) do
-        yield
-      end
-    end
-end
-
-class UsesWritingConnectionForFixtures < ActiveRecord::TestCase
+class MultipleFixtureConnectionsTest < ActiveRecord::TestCase
   include ActiveRecord::TestFixtures
-  self.use_transactional_tests = true
 
   fixtures :dogs
 
@@ -1454,6 +1425,13 @@ def test_uses_writing_connection_for_fixtures
     end
   end
 
+  def test_writing_and_reading_connections_are_the_same
+    rw_conn = ActiveRecord::Base.connection_handlers[:writing].connection_pool_list.first.connection
+    ro_conn = ActiveRecord::Base.connection_handlers[:reading].connection_pool_list.first.connection
+
+    assert_equal rw_conn, ro_conn
+  end
+
   private
     def config
       { "default" => default_config, "readonly" => readonly_config }