Backport #4542: test_threaded_flush sometimes fails

I found a way to reproduce this every time:

/home/yhara/temp/ruby_1_9_2_svn % svn di
Index: test/ruby/test_io.rb
===================================================================
--- test/ruby/test_io.rb        (リビジョン 31238)
+++ test/ruby/test_io.rb        (作業コピー)
@@ -1665,7 +1665,7 @@
       Thread.new {sleep 1; t.kill; p 'hi!'}
       t.join
     }.gsub(/^\s+/, '')
-    10.times.map do
+    100.times.map do
       Thread.start do
         assert_in_out_err([], src, [%q["hi!"]])
       end

Result:

/home/yhara/temp/ruby_1_9_2_svn % make test-all TESTS=test/ruby/test_io.rb
./miniruby -I./lib -I.ext/common -I./- -r./ext/purelib.rb  ./tool/runruby.rb --extout=.ext  -- "./test/runner.rb" test/ruby/test_io.rb
Loaded suite ./test/runner
Started
....................................................................................F........
Finished in 9.047658 seconds.

  1) Failure:
test_threaded_flush(TestIO) [/home/yhara/temp/ruby_1_9_2_svn/test/ruby/test_io.rb:1670]:
<["\"hi!\""]> expected but was
<[]>.

93 tests, 431 assertions, 1 failures, 0 errors, 0 skips

Test run options: --seed 49147
make: *** [yes-test-all] エラー 1

• Tracker changed from Bug to Backport
• Status changed from Open to Assigned
• Assignee set to yugui (Yuki Sonoda)

Hi, In trunk, test_threaded_flush was modified by r29022, and it's not backported to ruby_1_9_2 branch yet. Merging r29022 will fix this error.

• Project changed from Backport93 to Backport92
• Category changed from core to core
• Target version deleted (2.0.0)

The assigned project changed from backport93 to backport92. this fix is already applied to 1_9_3 branch.