go/types, types2: use correct predicate when asserting comma-ok types

go/types, types2: use correct predicate when asserting comma-ok types

While at it and unrelated, up-date testdata/manual.go sample file so
we can just copy its contents into a test file after debugging, without
fixing the date.

Fixes #66878.

Change-Id: Ie49a341b78d99bdc0f1a0ba1ca42fa2d3a807bd6
Reviewed-on: https://go-review.googlesource.com/c/go/+/580075
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>