path/filepath: remove IsLocal tests dependent on Windows version

path/filepath: remove IsLocal tests dependent on Windows version

Don't test IsLocal("nul.txt"), since whether this name is reserved
or not is version-dependent.

Change-Id: Ifff3edc77454e052080e325871c08bbba49e692c
Reviewed-on: https://go-review.googlesource.com/c/go/+/451222
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>