Fix multithread issue in the test of r35391

pipe() uses threads and its reader thread may work before the writer
writes "foob". On such case, the reader will raise error.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@35396 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_io.rb

@@ -1069,13 +1069,12 @@ class TestIO < Test::Unit::TestCase
 
   def test_read_nonblock_with_not_empty_buffer
     skip "IO#read_nonblock is not supported on file/pipe." if /mswin|bccwin|mingw/ =~ RUBY_PLATFORM
-    pipe(proc do |w|
+    with_pipe {|r, w|
       w.write "foob"
       w.close
-    end, proc do |r|
       r.read_nonblock(5, s = "01234567")
       assert_equal("foob", s)
-    end)
+    }
   end
 
   def test_read_nonblock_error