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 <0intro@gmail.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/os/stat_plan9.go

@@ -35,6 +35,10 @@ func fileInfoFromStat(d *syscall.Dir) FileInfo {
 	if d.Type != 'M' {
 		fs.mode |= ModeDevice
 	}
+	// Consider all files served by #c as character device files.
+	if d.Type == 'c' {
+		fs.mode |= ModeCharDevice
+	}
 	return fs
 }