tests: Improve fdt_resize() tests

We primarily test fdt_resize() in the sw_tree1 testcase, but it has
some deficiencies:

  - It didn't check for errors actually originating in fdt_resize(),
    just for errors before and after

  - It only tested cases where the resized buffer was at the same
    address as the original one, whereas fdt_resize() is also supposed
    to work if the new buffer is entirely separate, or partly
    overlapping

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