file(CREATE_LINK): Fix CMP0205 to not remove content behind directory symlink

When removing the destination directory link when it already exists
before making the new link, only the (destination) link should be
removed, not the (source) directory itself. When COPY_ON_ERROR is
specified and the destination is a directory, only then should CMake
remove it, since it could likely be placing a new directory there in
its place.

With this new behavior, adjust the conditions for the symlink test from
commit a73ddd2ddb (file(CREATE_LINK): Implement COPY_ON_ERROR for
directories, 2025-10-15, v4.3.0-rc1~599^2). The newly modified test
case should be run on systems that do and don't support directory
symlinks, with slightly different outcomes that are verified.

Fixes: #27747