cmd/go: list test packages even when the main package has an error

cmd/go: list test packages even when the main package has an error

List test packages (when list is run with -e) even when the main package
has an error. This is useful to get complete data for go/packages.

Fixes #37971

Change-Id: If6ba0270a319ea5e003d1ed8b1ad39e479e95509
Reviewed-on: https://go-review.googlesource.com/c/go/+/224944
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>