cmd/compile/internal/types2: list errors by default in TestManual

TestManual is used for debugging; in this case we usually want to
see error messages reported rather than checked against ERROR comments
in the provided files. Make this the default. Use the new -verify
flag to verify reported errors against ERROR comments.

With this change we cannot get an error list for the non-manual
tests, but that is usually not useful anyway because there are
usually many errors in those test files. Run those tests manually
instead.

Also, corrected -lang flag synopsys: it applies to all tests, not
just TestManual.

Change-Id: I56e0ea0583840fc3ea150d9ccfc330370b66191c
Reviewed-on: https://go-review.googlesource.com/c/go/+/315729


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>