Fix enumerating the root directory. (dotnet/corefx#27223)

Fix enumerating the root directory. (dotnet/corefx#27223)

* Fix enumerating the root directory.

Shouldn't trim the trailing separator when at the root. On Windows that makes
the path drive relative- which breaks most of the logic.

Add a test to catch this case.

* Add the Unix part of the fix.

Commit migrated from https://github.com/dotnet/corefx/commit/5b4ce203c563a20053f32f9901ea6b7dc67c8e37