[rubygems/rubygems] Revert "Disallow warnings to creep into the test suite"

This reverts commit 694e6afee769ffb3168a564ee7d315af2a934993.

It was failed on Windows and I'm unsure why. Also, the check seems
brittle since it could fail potentially fail because of warnings outside
of our control.

https://github.com/rubygems/rubygems/commit/908e9344dd

lib/rubygems/test_case.rb

@@ -289,10 +289,6 @@ class Gem::TestCase < Minitest::Test
   # or <tt>i686-darwin8.10.1</tt> otherwise.
 
   def setup
-    @orig_stderr = $stderr.dup
-    @captured_stderr = Tempfile.new("captured_stderr")
-    $stderr.reopen @captured_stderr
-
     @orig_env = ENV.to_hash
     @tmp = File.expand_path("tmp")
 
@@ -466,16 +462,6 @@ class Gem::TestCase < Minitest::Test
     end
 
     @back_ui.close
-
-    $stderr.rewind
-    err = @captured_stderr.read
-    assert_empty err
-  ensure
-    @captured_stderr.unlink
-
-    $stderr.reopen @orig_stderr
-    @orig_stderr.close
-    @captured_stderr.close
   end
 
   def credential_setup