DEV: Fix flaky twitter onebox behavior (#18141)

The order in which Onebox engines are loaded is not guaranteed. Occasionally during tests, the twitter engine would be loaded before the instagram engine, and cause the Instagram Onebox spec to fail due to the lack of `Onebox.options.twitter_client`.

This commit makes the load order of Onebox engines consistent, and fixes the issue in the twitter_status_onebox.

lib/onebox/engine.rb

@@ -9,7 +9,7 @@ module Onebox
     def self.engines
       constants.select do |constant|
         constant.to_s =~ /Onebox$/
-      end.map(&method(:const_get))
+      end.sort.map(&method(:const_get))
     end
 
     def self.all_iframe_origins

lib/onebox/engine/twitter_status_onebox.rb

@@ -11,7 +11,8 @@ module Onebox
       always_https
 
       def self.===(other)
-        !Onebox.options.twitter_client.twitter_credentials_missing? && super
+        client = Onebox.options.twitter_client
+        client && !client.twitter_credentials_missing? && super
       end
 
       def http_params