num cpu unit test fixes for FreeBSD.
cpuset -g can possibly output more
data than expected.

Fixes #25924

Change-Id: Iec45a919df68648759331da7cd1fa3b9f3ca4241
GitHub-Last-Rev: 4cc275b519cda13189ec48b581ab9ce00cacd7f6
GitHub-Pull-Request: golang/go#25931
Reviewed-on: https://go-review.googlesource.com/119376
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>