os: fix TestDevNullFile on Plan 9
os: fix TestDevNullFile on Plan 9

CL 102457 added TestDevNullFile. However, this
test is failing on Plan 9, because it checks
that /dev/null is a character device while there
are no special files on Plan 9.

We fix this issue by changing Stat to consider
all files served by the console device (#c)
as character devices.

Fixes #24534.

Change-Id: I1c60cdf25770358b908790b3fb71910fa914dec0
Reviewed-on: https://go-review.googlesource.com/102424
Run-TryBot: David du Colombier <ec8ae1e6620d4bab5e0e7aa2a6ae41aad63e3708@gmail.com>
Reviewed-by: Alex Brainman <54ba58c8441b5eff861de6b4e5ee46baf086b142@gmail.com>
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

Cofyc/go,golang/go,Cofyc/go,akutz/go,josharian/go,cloudflare/go,nawawi/go,akutz/go,cloudflare/go,CAFxX/go,golang/go,golang/go,josharian/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,golang/go,akutz/go,josharian/go,CAFxX/go,CAFxX/go,akutz/go,nawawi/go,CAFxX/go,Cofyc/go,CAFxX/go,akutz/go,Cofyc/go,nawawi/go,akutz/go,Cofyc/go,Cofyc/go,nawawi/go,josharian/go,cloudflare/go,golang/go,nawawi/go,josharian/go,golang/go,josharian/go,josharian/go,golang/go,cloudflare/go,Cofyc/go,josharian/go,cloudflare/go,nawawi/go,cloudflare/go,cloudflare/go,CAFxX/go,akutz/go,CAFxX/go,Cofyc/go,akutz/go,cloudflare/go