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