[dev.typeparams] cmd/compile/internal/types2: adjust tests, enable Testdata tests

Types2 uses a different test runner and has fewer/better
errors in some cases (error messages match the compiler).
Adjust the tests and enable them.

Change-Id: I74877f54a81a3918a80774452cef5bcaad8a98e6
Reviewed-on: https://go-review.googlesource.com/c/go/+/263631


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