[dev.typeparams] go/types: import api_test.go changes from dev.go2go

This CL imports tests for the go/types API from the dev.go2go branch.
Only parse type parameters for packages with a magic prefix, with the
rationale that while generics are in preview, we want existing
(non-generic) tests to exercise the default mode.

Change-Id: I8ae0d8769b997a8a93b708453a1afaecb262244d
Reviewed-on: https://go-review.googlesource.com/c/go/+/284693


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