dist test should not print (especially to stdout) during test
registration.  This confuses other tools interacting with dist using
dist test --list, etc.

Change-Id: Ie4f82c13e49590c23a7a235d90ddbc4f5ed81e0b
Reviewed-on: https://go-review.googlesource.com/12487Reviewed-by: Ian Lance Taylor <iant@golang.org>