cmd/go: parallelize part of loading test packages in list

cmd/go: parallelize part of loading test packages in list

load.TestPackagesAndErrors is given an optional done func() argument.
If set, load.TestPackagesAndErrors will perform part of its work
asynchronously and call done when done. This allows go list to run
testPackagesAndErrors so that the parallelizable parts of
TestPackagesAndErrors run in parallel, making go list -e faster.

Fixes #59157
Change-Id: I11f45bbb3ea4ceda928983bcf9fd41bfdcc4fbd9
Reviewed-on: https://go-review.googlesource.com/c/go/+/484496
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>