Shush up EM::Hiredis when running tests.

EM::Hiredis were spewing screenfuls of warnings when running the Action Cable tests.

Copied over the technique that shushes up faye-websocket in the client tests, so
we can reduce the noise ratio.

Note: there's still warnings spewed after tests have finished when EM::Hiredis shuts
down. I haven't been able to shush them up yet.

actioncable/test/subscription_adapter/evented_redis_test.rb

@@ -4,6 +4,17 @@ require_relative './common'
 class EventedRedisAdapterTest < ActionCable::TestCase
   include CommonSubscriptionAdapterTest
 
+  def setup
+    super
+
+    # em-hiredis is warning-rich
+    @previous_verbose, $VERBOSE = $VERBOSE, nil
+  end
+
+  def teardown
+    $VERBOSE = @previous_verbose
+  end
+
   def cable_config
     { adapter: 'evented_redis', url: 'redis://127.0.0.1:6379/12' }
   end