Only return true on Windows for Uri.IsUnc on file:// URI

Only return true on Windows for Uri.IsUnc on file:// URI

* System/Uri.cs: IsUnc returns true for file:/// URI on Windows only.
This makes us closer to Mac-behavior (see NCL tests) since the other
properties, e.g. LocalPath, match this. Also fix some exception text
where the values are checked by some test cases.

* Test/System/UriTest.cs: Adjust some test cases wrt Win32
* Test/System/UriTest2.cs: Adjust a test case wrt Win32