go.tools/go/ssa: add hook to testmain.go for proprietary (non-'go test') build systems.
go.tools/go/ssa: add hook to testmain.go for proprietary (non-'go test') build systems.

Users need only add an extra file to the package to specify
additional imports and initialization steps in testmain, to
match their build system.

LGTM=gri
R=gri
CC=golang-codereviews
https://codereview.appspot.com/120090043

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