GC before running test_copy_stream_socket7

it sometimes occurs NoMemoryError
http://c5664.rubyci.org/~chkbuild/ruby-trunk/log/20130529T033302Z.diff.html.gz

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40981 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_io.rb

@@ -660,6 +660,7 @@ class TestIO < Test::Unit::TestCase
   end if defined? UNIXSocket
 
   def test_copy_stream_socket7
+    GC.start
     mkcdtmpdir {
       megacontent = "abc" * 1234567
       File.open("megasrc", "w") {|f| f << megacontent }