Fix DirTestCase to run on the systems without "C:" drive.

Fix DirTestCase to run on the systems without "C:" drive.

"C:" drive doesn't need to exist under Windows, rely on HOMEDRIVE environment
variable defined in all recent Windows versions to get a valid drive letter
(still fall back to "C:" if the variable is not defined -- we could have use
wxFSVolume to find it then but this seems like an overkill).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65744 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775