increase timeout 10 to 15 on test_LSHIFT_neary_long_max

http://chkbuild002.hsbt.org/chkbuild/ruby-trunk/log/20140517T160021Z.fail.html.gz
http://chkbuild002.hsbt.org/chkbuild/ruby-trunk/log/20140517T200036Z.fail.html.gz

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45994 b2dd03c8-39d4-4d8f-98ff-823fe69b080e