go/types: update test case to exercise mutually recursive interfaces

Due to a typo, this test case was not actually exercising the bug
described in golang/go#33656. Update it to do so. Interestingly, the
comparison is now valid (as it should be) -- I suspect #33656 is
actually fixed.

Fixes #33656

Change-Id: If50a917f6477d8eb4f82f5a2a96bf5d9123ff0d4
Reviewed-on: https://go-review.googlesource.com/c/go/+/241263


Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>