[dev.typeparams] cmd/compile: cleanup ABI utils tests

This CL cleans a few minor nits with the ABI utils tests that are now
necessary because of cleanups that happened on master:

1. Initialize types.LocalPkg; this needs to be set for selector names
to be handled correctly.

2. In TestABIUtilsInterfaces, switch to using an exported identifier,
so it doesn't need to be package qualified.

3. While here, change the method result type from "untyped string" to
just "string". Constants are the only declared object that can ever
have an untyped type.

Change-Id: Iabed46594361a516317a1c2d20c3d59bdb519844
Reviewed-on: https://go-review.googlesource.com/c/go/+/333189


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>