go/ast: add test for Filter
go/ast: add test for Filter

Change-Id: Ia14659a9c44f9e1504eb88b5693932b9dd4bb286
Reviewed-on: https://go-review.googlesource.com/c/go/+/252939
Run-TryBot: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Trust: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Trust: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

cloudflare/go,nawawi/go,nawawi/go,nawawi/go,CAFxX/go,cloudflare/go,golang/go,nawawi/go,cloudflare/go,cloudflare/go,golang/go,golang/go,golang/go,CAFxX/go,cloudflare/go,golang/go,cloudflare/go,nawawi/go,nawawi/go,CAFxX/go,CAFxX/go,CAFxX/go,cloudflare/go,nawawi/go,CAFxX/go,CAFxX/go,golang/go,cloudflare/go,golang/go,golang/go,CAFxX/go,nawawi/go