The filepath.Abs function in windows did not call Clean as the
documentation claimed.  This change not only fixes that behavior but
also adjusts TestAbs to verify Abs calls Clean as documented.

Fixes #17210

Change-Id: I20c5f5026042fd7bd9d929ff5b17c8b2653f8afe
Reviewed-on: https://go-review.googlesource.com/32292Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>