cmd/go/internal/test: update documentation for the "go test" command

cmd/go/internal/test: update documentation for the "go test" command

The documentation referred to the package's source root as $GOPATH, which is no longer correct.

Fixes #64303

Change-Id: I2ea113497975726468d4ee4f85e2cfcbea9a76d6
Reviewed-on: https://go-review.googlesource.com/c/go/+/598235
Reviewed-by: Julian Dax <julian.dax@posteo.de>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>