go/types: add tests for conversion of non-constant untyped values

go/types: add tests for conversion of non-constant untyped values

This was fixed by CL 242084. Retroactively add some tests that would
have failed before the fix.

Also, remove some existing duplicate tests.

Change-Id: I95f7a215d4a9651ded6d739f89c574f33f573c60
Reviewed-on: https://go-review.googlesource.com/c/go/+/251397
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>