go/types: add test for interface method field type

go/types: add test for interface method field type

Add a test that would have detected the regression in #74303: interface
method fields should have a recorded type.

For #74303

Change-Id: Ide5df51cd71c38809c364bb4f95950163ecefb66
Reviewed-on: https://go-review.googlesource.com/c/go/+/683595
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Robert Findley <rfindley@google.com>