cmd/go: enable vet directive analyzer during 'go test'

cmd/go: enable vet directive analyzer during 'go test'

For #56986, run the new directive analyzer during 'go test',
to diagnose problems that would otherwise be missed,
like //go:debug appearing in the wrong place in a file
or in the wrong files.

Change-Id: I1ac230c3c67e58b5e584128e0ec6ff482cb225f9
Reviewed-on: https://go-review.googlesource.com/c/go/+/464135
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>