cmd/go: add test case for issue 64738

cmd/go: add test case for issue 64738

The straight revert in CL 551215 fixed this issue.
Add a test case to make sure we don't reintroduce it.

Test case copied from CL 550237 (by bcmills).

Fixes #64738.

Change-Id: I9654a1fd46fe1a1cc63ee6645a552ec21d720ad0
Reviewed-on: https://go-review.googlesource.com/c/go/+/551255
Reviewed-by: Michael Matloob <matloob@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Bryan Mills <bcmills@google.com>