net: use absDomainName in the Windows lookupPTR test helper

The real net code uses subtle heuristics to transform a domain name
to its absolute form. Since lookupPTR isn't checking that
transformation specifically, it should use the real code instead of
using a different heuristic.

Fixes #46882

Change-Id: I503357e0f62059c37c359cd54b44d343c7d5ab2a
Reviewed-on: https://go-review.googlesource.com/c/go/+/330249


Trust: Bryan C. Mills <bcmills@google.com>
Trust: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>