cmd/go: avoid internal errors on "go test -testwork"

cmd/go: avoid internal errors on "go test -testwork"

Fix up the package test harness to avoid errors of the form

  go_test.go:NNN: internal testsuite error: path(".") with no tempdir

when the "-testwork" flag is passed when running "go test".

Fixes #55874.

Change-Id: I76cc39902f51a62cb4cd0da8794b97d620fb4873
Reviewed-on: https://go-review.googlesource.com/c/go/+/434455
Run-TryBot: Than McIntosh <thanm@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>