cmd/go: make go list error behavior consistent in tests

"go list -test" constructs a package graph, then creates test packages
for the target. If it encounters an error (for example, a syntax error
in a test file or a test function with the wrong signature), it
reports the error and exits without printing the test packages or
their dependencies, even if the -e flag is given. This is a problem
for tools that operate on test files while users are editing them. For
example, autocomplete may not work while the user is typing.

With this change, a new function, load.TestPackagesAndErrors replaces
TestPackagesFor. The new function attaches errors to the returned test
packages instead of returning immediately. "go list -test" calls this
when the -e flag is set. TestPackagesFor now returns the same error as
before, but it returns non-nil packages so that "go list -test"
without -e can print partial results.

Fixes #28491

Change-Id: I141765c4574eae424d872eb9bf7dd63fdfb85efb
Reviewed-on: https://go-review.googlesource.com/c/go/+/164357
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

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