Simplify configuring of `ActionCable.server.config.logger` for actioncable tests

See `git grep "= Logger.new(nil)"`

actioncable/test/test_helper.rb

@@ -17,7 +17,7 @@
 
 # Set test adapter and logger
 ActionCable.server.config.cable = { "adapter" => "test" }
-ActionCable.server.config.logger = Logger.new(StringIO.new).tap { |l| l.level = Logger::UNKNOWN }
+ActionCable.server.config.logger = Logger.new(nil)
 
 class ActionCable::TestCase < ActiveSupport::TestCase
   include ActiveSupport::Testing::MethodCallAssertions