go/ast: move sortimports test directly to go/ast

go/ast: move sortimports test directly to go/ast

For some reason i have created a separate package instead
of using _test package. Let's move this test where it belongs.

Change-Id: Ib569ca433de1ef4e161b9d334125648e00b7d3c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/663555
Auto-Submit: Alan Donovan <adonovan@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>