Fixed renaming paths starting with "[a-z]:" being interpreted as drive names on Windows.

Fixes PICARD-663
Do not run the HiddenFileTest unit tests on Windows.

The unit tests only consider the string and whether any path component starts with a ".". On Windows the dotfiles are not used for hidden files so the tests fail. To have proper tests on Windows would require to have an actual folder structure with hidden files and folders.
Split the drive in replace_win32_incompat only on win32.

Also updated the tests, since this is now platform dependent.
Merge branch 'fix-win32-hidden-files-unit-tests' of https://github.com/phw/picard
Merge branch 'PICARD-663' of https://github.com/phw/picard