tests/subsys/fs/fat_fs_dual_drive: Add fs_dir_t type variable init.

The commit adds initializations of fs_dir_t variables in preparation
for fs_opendir function change that will require fs_dir_t object, passed
to the function, to be initialized before first usage.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>