Extend path_offset testcase for handling of duplicated separators

Paths with multiple '/' characters in a row (e.g. //somenode//somsubnode),
or trailing '/' characters (e.g. '/somenode/somesubnode/') should be
handled by fdt_path_offset(), and treated as equivalent to
/somenode/somesubnode.

Our current path_offset testcase doesn't check for these cases, so extend
it so it does.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>