os: reenable TestReaddirSmallSeek on windows

os: reenable TestReaddirSmallSeek on windows

TestReaddirSmallSeek should have been reenabled as part of
CL 405275, but didn't. Do it now.

Updates #36019

Change-Id: I5676eee4e63675d30e9d48ac708e72bd036b6aee
Reviewed-on: https://go-review.googlesource.com/c/go/+/458336
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: David Chase <drchase@google.com>