[ELFYAML] Provide explicit value for relocation addendums in the test

The `Addend` is an optional field of the `Relocation` YAML record. But
we do not provide its default value while reading it from a YAML file
and so it might keep uninitialized.

I am going to fix the code by a separate commit. We might either make
this field mandatory (at least for .rela sections) or specify 0 as
a default value explicitly.

llvm-svn: 227318