go/types, types2: always run tests with and without _Alias nodes enabled

go/types, types2: always run tests with and without _Alias nodes enabled

In manual mode, _Alias nodes are disabled by default and can be
enabled with a line comment (// -alias) at the start of a file.

Follow-up on feedback for CL 521956.

Change-Id: I937eb2e58e9e96fa6785ac45ca19e6328d2bd1fa
Reviewed-on: https://go-review.googlesource.com/c/go/+/541295
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>