Instead, add a small test verify Major, Minor and Mkdev using arbitrary
numbers.

Leave TestDevices on Linux as its major/minor numbers are guaranteed
stable.

Updates golang/go#25528
Updates golang/go#25535

Change-Id: Ic105211660d80f9b3508ca9d518e9fac3b7e3beb
Reviewed-on: https://go-review.googlesource.com/114396
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>