[rubygems/rubygems] Improve source list management testing

This approach better simulate that ["http://gems.example.com/"] is the
default list of sources, rather than ["https://rubygems.org/"].

https://github.com/rubygems/rubygems/commit/55130c259a

lib/rubygems/defaults.rb

@@ -13,7 +13,7 @@ module Gem
   # An Array of the default sources that come with RubyGems
 
   def self.default_sources
-    %w[https://rubygems.org/]
+    @default_sources ||= %w[https://rubygems.org/]
   end
 
   ##

test/rubygems/helper.rb

@@ -400,8 +400,9 @@ class Gem::TestCase < Test::Unit::TestCase
     Gem::RemoteFetcher.fetcher = Gem::FakeFetcher.new
 
     @gem_repo = "http://gems.example.com/"
+    Gem.instance_variable_set :@default_sources, [@gem_repo]
+    Gem.instance_variable_set :@sources, nil
     @uri = Gem::URI.parse @gem_repo
-    Gem.sources.replace [@gem_repo]
 
     Gem.searcher = nil
     Gem::SpecFetcher.fetcher = nil

test/rubygems/test_gem.rb

@@ -586,6 +586,7 @@ class TestGem < Gem::TestCase
   end
 
   def test_self_default_sources
+    Gem.remove_instance_variable :@default_sources
     assert_equal %w[https://rubygems.org/], Gem.default_sources
   end
 
@@ -1198,6 +1199,8 @@ class TestGem < Gem::TestCase
     Gem.sources = nil
     Gem.configuration.sources = %w[http://test.example.com/]
     assert_equal %w[http://test.example.com/], Gem.sources
+  ensure
+    Gem.configuration.sources = nil
   end
 
   def test_try_activate_returns_true_for_activated_specs