RFR(S): JDK-8232092 - Support for Windows drives created using subst

[Nhat Nguyen]

Hi nio-dev,

I'm sending this email as a follow-up to a conversation on an issue with
Windows's virtual drive:

https://mail.openjdk.java.net/pipermail/core-libs-dev/2020-July/067594.html
https://bugs.openjdk.java.net/browse/JDK-8232092

Since Alan pointed out that subst on Windows has never been fully supported by
the Java's file system module, I have done some additional testing with Windows
drives created using the subst command to see if we could identify other issues.

The test covers some common operations provided by java.nio.file.Files such as
creating, deleting, moving, copying, and creating symlinks of files in a virtual
drive; these APIs internally call into the native Windows APIs defined under
windows/native/libnio/fs/WindowsNativeDispatcher.c.

Unfortunately, I could not find any additional failures other than the unexpected
error code returned by GetVolumePathNameW causing Files.isWritable to produce
incorrect results; this could be fixed by Nikola's changes.

I understand that the test is by no means exhaustive and can miss other
cases, so I would really appreciate any further guidance on what else to test.

As a side note, the test contains three scenarios with symlinks which require
Administrator privileges to run. I'm not sure how these tests are normally run
using jtreg, so I decided to comment those out. Any suggestions are greatly
appreciated as well.

Webrev: http://cr.openjdk.java.net/~adityam/nhat/subst_drives/0/

Thank you,
Nhat Nguyen
Microsoft