cmd/go: disable automatic go vet -unreachable during go test of std

go test runs a limited number of vet checks by default.
In the standard library, we run more, both to get additional checking
for the standard library and to get experience with whether to enable
any others by default.

One that experience has shown us should not be enabled by default
is go vet -unreachable. When you are testing, it is common to want to
put an early return or a panic into code to bypass a section of code.
That often causes unreachable code. It's incredibly frustrating if the
result is an "unreachable code" error that keeps your test from completing.

Change-Id: Ib194e87759eb65f5a193d771a9880b38d2fd3ba9
Reviewed-on: https://go-review.googlesource.com/c/go/+/240550


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>