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 <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>
Reviewed-by: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go