test_io.rb: separate a test

* test/ruby/test_io.rb (test_closed_stream_in_rescue): run in a
  separated process.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57474 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_io.rb

@@ -3486,6 +3486,8 @@ __END__
     end
 
     def test_closed_stream_in_rescue
+      assert_separately([], "#{<<-"begin;"}\n#{<<~"end;"}")
+      begin;
       10.times do
         assert_nothing_raised(RuntimeError, /frozen IOError/) do
           IO.pipe do |r, w|
@@ -3503,6 +3505,7 @@ __END__
           end
         end
       end
+      end;
     end
 
     def test_write_no_garbage