Suppress leak of file descriptors

`Bundler.ui=` in `Gem::TestCase#setup` creates `Bundler::UI::RGProxy`
which inherites `::Gem::SilentUI` whose `initialize` opens `/dev/null`,
and assigns it to `Gem::DefaultUserInteraction.ui`.
After that, `Gem::TestCase#setup` forces to overwrite
`Gem::DefaultUserInteraction.ui` with a mock.
Thus, the instance of `::Gem::SilentUI` is not closed, which leads to
the leak.

This commit keeps `Gem::DefaultUserInteraction.ui` and manually close it
in `teardown`.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60194 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/rubygems/test_case.rb

@@ -236,6 +236,7 @@ class Gem::TestCase < MiniTest::Unit::TestCase
     @fetcher     = nil
 
     Bundler.ui                     = Bundler::UI::Silent.new
+    @back_ui                       = Gem::DefaultUserInteraction.ui
     @ui                            = Gem::MockGemUi.new
     # This needs to be a new instance since we call use_ui(@ui) when we want to
     # capture output
@@ -420,6 +421,8 @@ class Gem::TestCase < MiniTest::Unit::TestCase
     Gem::Specification._clear_load_cache
     Gem::Specification.unresolved_deps.clear
     Gem::refresh
+
+    @back_ui.close
   end
 
   def common_installer_setup