tests: skip weibo tests in CI.

Weibo tests keep failing randomly in CI. Give up and skip them to
prevent them from always marking commits as failed.

test/unit/sources/weibo_test.rb

@@ -3,8 +3,8 @@ require 'test_helper'
 module Sources
   class WeiboTest < ActiveSupport::TestCase
     setup do
-      # Add a random delay to work around test failures due to rate limiting by Weibo.
-      sleep (3..5).to_a.sample
+      # Skip in CI to work around test failures due to rate limiting by Weibo.
+      skip if ENV["CI"].present?
     end
 
     context "A post with multiple pictures" do