runtime: add more debug info to flaky TestNumGoroutine
This has been flaking on the new OpenBSD 5.8 builders lately:
https://storage.googleapis.com/go-build-log/
808270e7/openbsd-amd64-gce58_61ce2663.log
(as one example)
Add more debug info when it fails.
Updates #14107
Change-Id: Ie30bc0c703d2e9ee993d1e232ffc5f2d17e65c97
Reviewed-on: https://go-review.googlesource.com/18938
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>