go/types: add example test for type inference

go/types: add example test for type inference

This is a port of CL 308973. The only change is to remove a TODO at
inference.go2:100 to improve the error position. The go/types error
position is fine.

Change-Id: Ibf61f3458adde91dec9c7531cbd892ca654a5497
Reviewed-on: https://go-review.googlesource.com/c/go/+/314593
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>