cmd/go: apply "go vet" to test files

In earlier versions of Go the "go vet" command would run on regular
source files and test files. That was lost in CL74750.  Bring it back.

This required moving a chunk of code from internal/test to
internal/load. The diff looks big but the code is unchanged.

Fixes #23395

Change-Id: Ie9ec183337e8db81c5fc421d118a22b351b5409e
Reviewed-on: https://go-review.googlesource.com/87636


Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>