test_gem_remote_fetcher.rb: fix leaked threads

* test/rubygems/test_gem_remote_fetcher.rb (stop_servers): kill
  server threads to fix leaked threads.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46636 b2dd03c8-39d4-4d8f-98ff-823fe69b080e