of: unittest: Fix memory leak in unittest_data_add()

In the Linux kernel, the following vulnerability has been resolved:

of: unittest: Fix memory leak in unittest_data_add()

In unittest_data_add(), if of_resolve_phandles() fails, the allocated unittest_data is not freed, leading to a memory leak.

Fix this by using scope-based cleanup helper __free(kfree) for automatic resource cleanup. This ensures unittest_data is automatically freed when it goes out of scope in error paths.

For the success path, use retain_and_null_ptr() to transfer ownership of the memory to the device tree and prevent double freeing.