Fix restore command absolute path error for Windows

[MattKotsenas]

When I run dnvm restore on Windows I get the following error:

Unhandled exception. System.ArgumentException: Path `C:/Projects/dnvm/global.json` must be absolute (Parameter 'path')
   at Zio.UPathExtensions.AssertAbsolute(UPath, String) + 0x1a7
   at Zio.FileSystems.FileSystem.ValidatePath(UPath, String, Boolean) + 0x66
   at Zio.FileSystems.FileSystem.FileExists(UPath) + 0x2e
   at Dnvm.RestoreCommand.<Run>d__1.MoveNext() + 0xa9
--- End of stack trace from previous location ---
   at Dnvm.Program.<Dnvm>d__3.MoveNext() + 0x4a7
--- End of stack trace from previous location ---
   at Dnvm.Program.<Main>d__1.MoveNext() + 0x259
--- End of stack trace from previous location ---
   at Dnvm.Program.<Main>(String[] args) + 0x28

This is because on Windows, UPath(Environment.CurrentDirectory) returns a non-absolute Windows-style (i.e. C:/) path. Calling ConvertPathFromInternal should address these issues.

While getting tests up and running, I updated ManifestTests and SelectTests to use the same Dnvm.Tests namespace as the other tests. I also made the chmod call conditional on platform in RunUpdateSelfInstaller so it works cross-platform.

[agocke]

LGTM