Fix directory enumeration callback in Simple provider

[narasamdya]

Fix directory enumeration callback by properly following the documentation:

https://docs.microsoft.com/en-us/windows/win32/api/projectedfslib/nc-projectedfslib-prj_get_directory_enumeration_cb#return-value

[erikmav]

[cgallred]

The CI failures in the 2022 version are from the symlink tests. They fail because the .NET Framework 4.8 version of the fixed Simple provider trips over the platform bug. The .NET Core 3.1 one works fine. Evidently Framework and Core enumerate differently, with Framework using a buffer that is small enough to hit the platform bug.

The 2019 version doesn't run the symlink tests at all, since symlink support isn't in that version of ProjFS.