cmd/go: go test std should give a clearer error with GO111MODULE=off #81364

Go version

go version go1.27.1 darwin/arm64

What did you do?

Prior to filing this issue, I forgot that it was already decided and documented that testing the Go standard library with GO111MODULE=auto or GO111MODULE=off is not supported (see go help gopath and issue #69202). Consider what happens if someone attempts to do it anyway:

$ go test net/http
$ GO111MODULE=off go test net/http

What did you see happen?

$ go test net/http
ok  	net/http	13.379s
$ GO111MODULE=off go test net/http
< 3186407 bytes of output, which are unhelpful as testing standard library packages
  with GO111MODULE111=off is unsupported and known to produce false positive signal >

What did you expect to see?

$ go test net/http
ok  	net/http	13.379s
$ GO111MODULE=off go test net/http
go: modules disabled by GO111MODULE=off; see 'go help gopath'

Or some better phrasing - see related issue #66080 about go get error clarity.

Implementing this might be tricky since it'd have to be done without also breaking the ability for cmd/go tests themselves to test this mode (given proposal #60915), but if it can be done somehow it may reduce possibility of confusion in the future.

CC @matloob (via https://dev.golang.org/owners).