[dev.typeparams] cmd/compile/internal/syntax: don't panic when providing -verify

The -verify flag is used to verify idempotent printing of syntax
trees. While syntax tree printing is not actively used at the
moment, the verification code still shouldn't panic.

Fixed the cause for the panic (after reading from a bytes.Buffer
that buffer is empty and so doesn't compare to the unread buffer),
and replaced the panic with a test error.

Added a test that makes sure the code invoked by -verify is run.

Change-Id: I38634ed7cfa8668deb0ea2ee9fb74a8f86cfc195
Reviewed-on: https://go-review.googlesource.com/c/go/+/278477


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