cmd/cgo/internal/testerrors: unskip TestPointerChecks on Windows
All TestPointerChecks cases can be executed on Windows with some minor
tweaks.
It seems that the skip has been cargo-culted from when cgo error tests
were first added in https://codereview.appspot.com/
13498046.
Change-Id: I39cd05f4c90965b669d2b403f7fcd9dd9c69016c
Reviewed-on: https://go-review.googlesource.com/c/go/+/514296
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>