go/types: use commentMap to collect error comments

go/types: use commentMap to collect error comments

Adjust the testFiles function to use the new commentMap
function. This makes it possible for testFiles to match
the types2.TestFiles logic more closely.

For #51006.

Change-Id: I6c5ecbeb86d095404ec04ba4452fb90d404b8280
Reviewed-on: https://go-review.googlesource.com/c/go/+/456137
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>