go/types: add tests for AssignableTo and ConvertibleTo
These exported functions are mostly trivial wrappers, but do make
certain assumptions about how the underlying Checker APIs can be called.
Add some simple tests.
Change-Id: I68e9ae875353c12d118ec961a6f3834385fbbb97
Reviewed-on: https://go-review.googlesource.com/c/go/+/241262
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Robert Griesemer <gri@golang.org>