Revert "cmd/go: move automatic testing.Init call into generated test code"

This reverts CL 176098.

Reason for revert: added complexity, but did not completely fix the
underlying problem. A complete solution would not be worth the
complexity, and as a partial solution this is probably not worth the
complexity either.

Updates #31859

Change-Id: Ifd34c292fd1b811c60afe3c339e5edd3f37190c8
Reviewed-on: https://go-review.googlesource.com/c/go/+/186817


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Caleb Spare <cespare@gmail.com>