go.tools/oracle: disable TestOracle test on windows (fixes windows build)
go.tools/oracle: disable TestOracle test on windows (fixes windows build)

windows does not have required /usr/bin/diff program

R=golang-dev, r
CC=adonovan, golang-dev
https://codereview.appspot.com/13353047

d0f/go-zh.tools,webfd/go-zh.tools,8limbs/gotools,webfd/go-zh.tools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,8limbs/gotools,skyportsystems/go.tools,skyportsystems/go.tools,skyportsystems/go.tools,webfd/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,skyportsystems/go.tools,8limbs/gotools,webfd/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,skyportsystems/go.tools