testing: Add testing.MainStart function.
testing: Add testing.MainStart function.

This patches in the Go 1.4 function, for convenience for
people using the Go 1.4 go tool with mainline gccgo.

R=iant
CC=gofrontend-dev
https://golang.org/cl/190870043

qskycolor/gofrontend,anlhord/gofrontend,anlhord/gofrontend,golang/gofrontend,qskycolor/gofrontend,golang/gofrontend,qskycolor/gofrontend,qskycolor/gofrontend,anlhord/gofrontend,golang/gofrontend,qskycolor/gofrontend,anlhord/gofrontend,anlhord/gofrontend,anlhord/gofrontend,golang/gofrontend,qskycolor/gofrontend,qskycolor/gofrontend,anlhord/gofrontend