runtime/trace: fix broken TestSubscribers

runtime/trace: fix broken TestSubscribers

Currently we don't break out of the loop on a failure. This is
leading to timeouts trying to parse a broken trace over and over,
forever.

But there's another issue where when we try to dump the trace, we pass
only the unread portion of the bytes.Buffer.

Change-Id: I1a338eea08eaf7f592fb7dd2a736a6fe0728c62d
Reviewed-on: https://go-review.googlesource.com/c/go/+/729320
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>