os: add test for Windows Dedup volumes

[alexbrainman]

In recent CL 143578, I was going to remove some code that I thought did nothing. My change would have made all files on Windows Dedup volumes ( https://docs.microsoft.com/en-us/windows-server/storage/data-deduplication/overview ) look like symlinks.

Luckily @kolyshkin pointed my mistake ( #22579 (comment) ). I built Windows instance on Gogle Cloud with extra disk attached as D: drive and with Dedup option on, to test my CL.

I wonder, if we can have one of our Windows builders have small extra disk with Dedup on. Then we could write some appropriate tests that run on that builder.

/cc @bradfitz @johnsonj

Alex

[bradfitz]

/cc @dmitshur