Join a thread.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46357 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/io/nonblock/test_flush.rb

@@ -29,7 +29,7 @@ class TestIONonblock < Test::Unit::TestCase
     w << "a" * 4096
     result = ""
     timeout(10) {
-      Thread.new {
+      t0 = Thread.new {
         Thread.pass
         w.close
       }
@@ -44,6 +44,7 @@ class TestIONonblock < Test::Unit::TestCase
         # ignore [ruby-dev:35638]
       end
       assert_nothing_raised {t.join}
+      t0.join
     }
     assert_equal(4097, result.size)
     true