If GO_TESTING_GOTOOLS is set in the environment, permit tests using
gccgo to run the go tool. Like GO_BUILDER_NAME, this should not be set
normally. But it is needed when testing the go tool itself, and will
be set by the gotools Makefile in a future CL.
Reviewed-on: https://go-review.googlesource.com/45693
From-SVN: r249195
