go/types, types2: add additional tests using core types during unification

go/types, types2: add additional tests using core types during unification

This change adds tests that use a type parameter's core type during
function argument type inference, not just during constraint type
inference.

Also, fix a typo in a comment.

For #50755.

Change-Id: I0c3196bdce5338341e0b6dfd7c63efb2e43ace25
Reviewed-on: https://go-review.googlesource.com/c/go/+/385376
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>