TestHostname was re-enabled in CL 10753.
However, on Plan 9 the hostname is not obtained
by executing a "hostname" command, but by reading
the #c/sysname file.

Change-Id: I80c0e303f4983fe39ceb300ad64e2c4a8392b695
Reviewed-on: https://go-review.googlesource.com/11033
Run-TryBot: David du Colombier <0intro@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>