go.tools/go/types: -files flag for testing one-off packages
go.tools/go/types: -files flag for testing one-off packages

Removed special case for testdata/test.go file in favor of
a simpler, more flexible, and explicit flag for one-off test
packages.

R=adonovan
CC=golang-dev
https://codereview.appspot.com/10618044

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