cmd/go: rename renamed testing import
_go_testing is equally unlikely and much less concerning
than testing_xxxxxxxxxxxx if it appears in an error message
(as it does, for example, in https://storage.googleapis.com/go-build-log/
0d543f89/linux-amd64_3467a10e.log).
Change-Id: I45dc3ebe2d3b6c9e53273cd21782ee11a53f5edb
Reviewed-on: https://go-review.googlesource.com/c/go/+/177197
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>