Build could use the package comment from test files to populate the .Doc
field on *Package.

As go list uses this data and several packages in the standard library
have tests with package comments, this lead to:

$ go list -f '{{.Doc}}' flag container/heap image
These examples demonstrate more intricate uses of the flag package.
This example demonstrates an integer heap built using the heap interface.
This example demonstrates decoding a JPEG image and examining its pixels.

This change now only examines non-test files when attempting to populate
.Doc, resulting in the expected behavior:

$ gotip list -f '{{.Doc}}' flag container/heap image
Package flag implements command-line flag parsing.
Package heap provides heap operations for any type that implements heap.Interface.
Package image implements a basic 2-D image library.

Fixes #23594

Change-Id: I37171c26ec5cc573efd273556a05223c6f675968
Reviewed-on: https://go-review.googlesource.com/96976
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Ian Lance Taylor <iant@golang.org>