Previously go list -test <pkg> would return pkg and, if it exists,
pkg.test, the test main package. This change will also list the
two test code packages (if they exist) that contain testing functions,
<pkg> [<pkg>.test] and <pkg>_test [<pkg>.test].

These packages which contain testing code are usually the packages
go list users desire to know about, so they should be surfaced
in go list -test.

See the discussion at
golang.org/cl/123635#message-5befbc66663063fb7247645a02ab1327a681e362
for more context.

Change-Id: I7170b539d02b548c050ac54048735ed785f47389
Reviewed-on: https://go-review.googlesource.com/126475
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>