test_io.rb: avoid OOM

* test/ruby/test_io.rb (TestIO#test_copy_stream_socket7): reduce
  memory usage so the worker process will not die by OOM.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61961 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_io.rb

@@ -869,7 +869,8 @@ class TestIO < Test::Unit::TestCase
               s1.close
               IO.select([s2])
               Process.kill(:USR1, Process.ppid)
-              s2.read
+              buf = String.new(capacity: 16384)
+              nil while s2.read(16384, buf)
             end
             s2.close
             nr.times do