Change libufdt test cases to compare with libfdt

The dtc adds a new function fdt_overlay_apply() to support DTO.
The original libufdt test cases used /include/ trick to compare
the test case result. Change to compare with the result of
fdt_overlay_apply().

Bug: 35495792
Test: run test scripts (see libufdt/tests/README)
Change-Id: Ied68c09c479ab09025574eb97c5ef3adf58253c4