Allow custom Redis instance for tests

You probably don't want the tests to use the same Redis you use for any
app development.

Maybe you want to use Docker with custom port:

    docker run --rm -it -p 16379:6379 redis:6

sentry-resque/spec/spec_helper.rb

@@ -4,6 +4,9 @@
 require "resque"
 require "resque-retry"
 
+# Allow customization of the Redis instance used for tests
+Resque.redis = ENV["REDIS_URL"] if ENV.key?("REDIS_URL")
+
 # To workaround https://github.com/steveklabnik/mono_logger/issues/13
 # Note: mono_logger is resque's default logger
 Resque.logger = ::Logger.new(nil)